I'm thinking of getting an od auto transmission for my 66 /6 Valiant. What trans should I be looking for and where could I find one?

There is no overdrive automatic transmission that will bolt into a '66 Valiant without a lot of additional parts and undercar bodywork (floorpan modifications, welding, new crossmember, etc). It is not impossible, but it is a very big, very expensive job.

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Too many people who were born on third base actually believe they've hit a triple.

If I were trying to install an overdrive automatic behind a slant six, I would get an A500 transmission from a late 80s or early 90s Dodge truck or van and one of the 60s era slant six to smallblock transmission adapter packages that Chrysler made to mate a slnt six to a smllblock transmission. As Dan said, you will need to mofidy the floor pan, trans cross member, driveshaft, add cooling lines, add a trans oil to air cooler if your radiator does not have the stock oil to water cooler, add the kickdown linkage, add the hydraulic switches to control the overdrive function, and likely more. It can and has been done, but you need to get all your research done. You might also look into fabricating an adapter plate to mate a Brand X transmission such as a 700R4 to the slant six.

Do you really want an automatic? You can do a much easier install of a 833 OD. After the floor hump is installed it is practically a bolt in.

The A500 does not have great support in the aftermarket. Lou Madsen is the guy to speak to this. He ran one in his Dart for many years, and he can relate some of the issues with that swap.

You can buy an adapter to bolt the GM transmission into a slant 6 too.

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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

If you want an automatic, but are not comfortable with major mods to install an OD unit, you might consider a later slant six 904, with a lock up converter. That is a much easier conversion.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

Thank you everybody, guess i'l start looking for a 904 !!!

For reference, this FABO thread includes a photo-illustrated account of modifying a '66 Dart to accept an overdrive automatic transmission. The car has a V8 engine, so the undercar mods were easier (transmission not moved rearward by necessary adapter plate as with a Slant-6)—gauge from there.

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Too many people who were born on third base actually believe they've hit a triple.

Last year I bought a 64 barn find /6 out of a truck complete with a PB 727 just for the /6 to SB trans adapter stuff. I sold the engine to a guy with a 64 valiant convertible who was wanting to replace an original 170 and I sold the trans as a core to a friend of Zilla's. That setup is so hard to find, , I couldn't pass it up.
Last summer I bought a very rusty 91 Dakota that I stripped for parts and kept the a500 out of it possibly for my 85 1/2 ton. I'm not sure which way I want to go with this truck, originally I was dead set on keeping it a 727 as that's original and still working but leaks as it sits. I think it's either the shift shaft seal or the o rings on the rear band anchor shaft. But it shifts so well when it's full of fluid.
I wasn't looking but first I found ( more like was gifted) EVERYTHING to convert to an 833OD. So I was "sure" I was going to that. That was before the Dakota fell into my lap and then this SB adapter setup did the same
All I've done is to confuse myself.
Had i actually been looking for any of those additional parts I wouldn't have found them for less than what they weigh in gold bouillon. Things find me lately when I'm not looking.
What I'm NOT doing is regression from a 727 to a 904 like so many were trying to convince me to do, I actually had one of those too and gave it away...

Thank You Everybody !!!