They will certainly help but with 3.91 or lower gears your not going to be able to drive very fast on the road without overdrive. If you had a manual transmission and overdrive I would go with 4.30's

There is a lot of info for your friend on here already, but if he plans to tow that much weight, and do it on a regular basis, I think he needs more HP. Either a stronger slant 6 or a V-8. Then, if he does get the Slant 6 motor where he needs it, he still has to address burning up the 904. Then the rear end needs lower gears.....etc. You get the picture. To make this truck what he wants will cost some serious change. He may be better off and cheaper just buying a truck set up to do what he wants.

I tow my race car about 30 miles to the track with my 100HP 85 truck with an autoatic and 3.21 gears. The car and trailer weigh considerably less than you are talking about towing. Probably 4200-4400#. It tows ok on level to near level ground. But the first slight grade that you hit you are searching for 2nd gear to keep running at 50 MPH. I wouldn't want to think about traveling up a mountain on an interstate that may be 5 miles long. I'd likely get run over while going 40 MPH up the grade.

I would feel much better if I was towing with a 4 speed and had another 50+ horsepower coming out of my Slant.

Just another opinion for the pile,

Rick

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2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6

Ok, i will share this with him. He doesn't have internet which is why he asked me to check. I told him that if it couldn't be done than y'all would know why. Thank you all for your patience and actually useful input, much better than many other sources. Knowing why hecan't do certain things with certain parts is going to help me explain it to him convincingly. It's also nice to know the sorts of things we can do to help tow better, rear axle, cooling the transmission, etc. . Thanks again.

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-Christopher

"Gray Ghost"-1970 Dart.
"Frederick II"-1994 Geo Metro.

In my opinion, the priority of modifications should be as follows:

(1) Rear axle gear ratio- at the very least go to 3.9, but a 4.1 or numerically higher wouldn't be a bad idea
(2) Transmission. Worst choice-Severe duty 904 rebuild, middle ground choice- built 727, best choice manual four speed with overdrive. If you are keeping an automatic, definitely install a big trans oil cooler.
(3) Engine modifications. mill the head for improved dynamic compression ratio, keep the one barrel carb and stock valves, bump up to the Erson RV10 or 15 cam grind.

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Casually looking for a Clifford hyperpak intake for cheap.

You could add a transfer case on the back end of the tranny to get you lower gearing.............

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

My vote would be in this order:

Slant six NP-235/435 granny gear sever duty tranny for hauling loads with the 11" clutch and use a 3.55/3.73 gear (these have a very granny 1st gear...deeper than the 3.09-1st the A-833OD ever had). I'll have to thank Midi for running a few yard of bark dust for me in his D-150...1 barrel/NP tranny...it didn't even flinch while loaded.

Second Choice A-727 with aux tranny coolers

3rd choice A-833OD and 4.56's (assume 235 tires?), the aluminum case will wallow out the countershaft hole lugging a heavy vehicle around, much less towing anything...for best effect and less wear on this tranny 3rd gear is the only key since it locks the main and input shaft together...and using a 1975 iron case main body.
The 10" stocker clutch won't hold up to the abuse as well either if getting a load started...This tranny will get him good mileage with the 4.10/4.56 gearing available over the counter...

4th Choice A-904T/LA/Stage 2,etc...about as comparible to the A-833OD...

-D.Idiot

Here's something else... I used my loaded, to just over it's GVW, '74 D100 to tow a '71 Dart from Sunnyvale, CA to Portland Oregon. The truck had a 225, A727 with a cooler and 3.55 gears. It was originally a 3-speed manual trans. The engine was in quite good condition, but stock. There were many grades in the mountains that were 25 MPH in low. Acceleration from a stop was abysmal. But what really bothered me was the lack of braking power. Sure the truck had disc brakes, but the dolly had nothing. It was dangerous and I would not repeat the adventure.

I replaced that D100 with a B300. The van had a 318, 727 and 4.10 gears. It towed much better from both power and braking standpoints. I briefly had a '96 Ram 2500 with a Cummins putting out about 275hp and 650 ft/lbs torque. Even though it would only do 87 MPH in overdrive with 4.10 gears I was ruined by the ease with which it towed.

Unless your friend lives in Kansas I would not pull 7000 lbs more than once with a 225 and then only if I had to.

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Joshua

Once you get the tow vehicle and trailer moving, you have to be able to stop it reasonably well also. Disc brakes up front are a must and ideally a full floating rear axle to sustain the weight is the best way to go. Towing can be done with a car type rear using semi floating axle, but that's not the best for towing. Remember this: there is no replacement for displacement. Use the largest engine available for towing, and you will not need a crazy high numerical axle ratio. Something like 3.92 will be sufficient, but any Slant Six will be working hard. Try something like a 360 LA block, or a 383-440 RB..Now you will have the towing "grunt" that a Slant Six can only dream about. If you are going to tow anything, get serious. Don't play games with a weakling. Slant Sixes are great in light weight vehicles but they don't make great towing vehicles. Maybe if you are towing a pop up camper trailer they are OK, but not for towing a trailer with a car on it.

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1956 Dodge 1/2 ton w/318,..1952 Ford 1/2 ton w/Hemi,..1940 Plymouth Business Coupe w/flathead six, dual carbs, and headers

The good news is that i showed him the thread and he has decided that he will accept your better judgement. The bad news is that now i am curious: does anyone happen to know what transmission and rear ratio was used in an aircraft tug? This last question is purely to satisfy my curiosity.

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-Christopher

"Gray Ghost"-1970 Dart.
"Frederick II"-1994 Geo Metro.