I am wanting to switch my 65 stepside over to an automatic from the 3spd on the column. I have 391 gears. Which would be the better way to go, the 904 or the 727? Are they both 3 speed autos? Does one "rob" power more than the other?
And should I change the rear gears to a lower numerical ratio?
Thanks for any advice you may give.

Both the 904 and 727 are 3 speed autos. The 727 has more parasitic power loss than a 904, but can handle more power and torque than a 904 (when both are in stock form)

The 904 is smaller and lighter than a 727, and to bolt a 727 to a slant you need a rare adapter plate or an even more rare slant 727 bellhousing. (A reproduction of the plate is in the works, check the post "Auto OD conversion available soon" down a few posts in this forum. If you go this route you may want to consider the newer A500 or A518 4 speed auto tranny's)

Unless availability and/or price between the two are factors in your area, a 904 is going to be your best option.

Whether or not you need to change rear gears depends on what the truck will be used for. If you do any highway driving, your engine will be screaming at 65 mph with those gears and no OD.

-S/6

What years and models did the newer trannys come in, ie the a500 and 518.

Try this one. If you want a 727, go to a junkyard and look for one in Dodge pickup that has a Slant. A 727 can be identified by the bulge where the dipstick tube enters the pan. My uncle has a 82 D-150 that came from the factory with a 225/727 combo. If your truck has a 3.25 gear ratio or higher, change it. You won,t be able to pull a greasy string out a cat's butt with a slant and that rear. I recommend 3.55 gears. If your truck has a 8 3/4 rear, you can just simply change the chunk out with another 8 3/4 of the correct ratio.

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82 D150-225/727
02 Dakota-3.9/5 speed
87 GMC C7000-8.2 Detroit Diesel/5+2