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JAH's 1968 GTO

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55/65 motor swap

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Hi Marv. welcome to FP.

I am a little confused by your question. The short answer is yes, the transmission bolt pattern is the same.

However, are you planning on re-using the stock Roto-Hydramatic transmission from the '55 and hooking it up to the newer 326? If so, I would advise against it. The original Pontiac 287 V8s were notorious (even by yesteryear's standards) for not being able to get out of their own shadow. They produced a whopping 180 HP/265 lb-ft so the transmission didn't need to hold up to much torque. The newer 326 thru 455s can be real HP/torque monsters if they are built properly with the right selection of aftermarket parts. So I would plan on getting a newer Buick/Olds/Pontiac bolt-pattern TH350, 400, 200 4R (overdrive) or a rebuilt Muncie or Saginaw manual unit, that can handle the torque your new motor is going to produce and there is plenty of aftermarket support for these transmissions versus the old Roto-Hydramatic.

Another thing you have to consider is replacing the original stock '55 radiator. The early Pontiac V8s had a reverse flow cooling system, as compared to today's conventional engines. Therefore, the original '55 radiator won't work with the 326, since the 326 flows conventionally.

Edited by Frosty
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Hope this is the right place,  the 326 I got, cheap, has a ton of rust in #1 cyl. so it's rebuild or look for another block that prob needs rebuild too. Hoping the bellhousing fits the 326. Has anyone tried to use the 55 bell, clutch linkage, fork, throw out bearing, etc with a 4 spd?

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Hi again Marv. Again my question is the 4-speed originally to the '55 or is it a 60s-era or newer Saginaw, Muncie, or Borg-Warner style 4-speed? If it isn't the original '55 transmission, I would go with a modern OEM/OER replacement bell housing, fork, clutch, and throw out bearing that fits the 326 and 4-speed. The clutch linkage still all connect up. You may have to change the adjustment rod a little. Most of the modern parts should be readily available at most part stores or online catalog stores., Rockauto.com, etc.

My theme is this, since you are not going with an all original, numbers matching engine in the car, who cares if the transmission isn't original either at this point? So if you are going to build up a reliable 326, 400 or 455 engine, why  not build a reliable transmission that fits that motor too.

Edited by Frosty
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Trying to use the parts I have on hand and not throw a lot of money at the project. Will need that for some of the parts that are missing and interior.,  60-70s 4 spd. Floor shift.  Just hoping to use bell and linkage with a new clutch.  I have a lot of Chevy parts.  It looks like s chevy throwout bearing would fit and slide over the snout on the 4spd. First need to see if 326 is rebuildable.  Had a running 389, but someone bought it out from under me.Thanks!

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