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I have a 56 GMC 3/4 ton with a pontiac 317 4 spd. I'm wondering could an automatic tranny 700 be matched up? I suppose I could get a auto from 50's but would like newer 4 spd. any advice would be appreciated. thanks

I don't think the 700R4 was built with anything other than the Chevy bolt pattern, but others will chime in soon. Could use a 2004R behind it, it has the dual bolt pattern. Beef it up a little and it will do you a good job.

The 2004R would be waaaayyyy to light for a 3/4 ton. Even a 700R4 is a tad on the light side, GM only used it on the light duty 3/4 tons, which aren't much more than a HD 1/2 ton.
If you really want an automatic that's properly mathced to a 3/4 ton truck, look into the 4L80, which is standard equipment in all GM 3/4 and 1 ton trucks '07 and newer (and was optional from '94-'07). There's conversion kits out to deal with the electronics, the only real isse would be adapting it to fit the Poncho engine. May need to have an adapter palte fabricated, not only to mathc the bolt pattern, but also to provide a rear engine mounting.

The rear of the early poncho motors is different from the rear of the later poncho motors. If you were going to run an automatic in your 56 jimmy with that motor it would be easiest to run a HydraMatic. You would have to reinvent the wheel to use a 700R4 or a th350 or th400.

Poncho motors changed around 1961. 1955-1960 bellhousings and transmissions should interchange.

Actually, the 2004R can be beefed up quite nicely. Got a bad rap early on, but I have had very good results with them. Could handle whatever the 317 could throw its way if built right. But the bolt pattern may very well kill that idea any way.

I don't know what year the bell housing pattern changed, but I had a 1960 Pontiac with a 389 that had the standard BOP bell pattern. It had a 4 speed automatic in it, but it has been so long I don't remember what it was.

But I see lots of folks sell the 2004R short, when it can be a very stout transmission.

While I have seen 2004Rs that have been built up to be really tough, they just simply are not rated for the weight capacity that a truck would see. even the S10s only used them on 4 cylinder models. It would make much more sence to use a trans that's already built for the application than dump alot of money into one that may not work.
Either way, the bellhousing issue does kill the idea of a late model trans. I kind of suspected that there was a bolt pattern issue, but couldn't remember if it was Pontiac or Olds that it applied to.
I'm assuming that you're looking to use a late model trans to get the overdrive, but is there any reason you're wanting an automatic? A manual overdrive may be easier to adapt, either using a 5 speed from a later truck, an MY6 from an '80s Chevy, or perhaps a standard 4 speed with either a Gear Vendors or Ranger overdrive.

http://www.wilcap.com/pontiac.html#287350

Found this in another topic

Hope it helps!

Wicked, I didn't know those exhisted. Still, My preference is for a manual transmission.