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From what I understand, all Pontiac V8's from the 326 to the 455 have the same outer dimensions, but would I have to change anything if I swapped a 400 or 455 into a '66 326 car? Motor mounts, etc? I'd rather have a 389 or 400 but I'm leaning toward a 455 since they're bigger AND cheaper.

The trans is a 2spd, probably an ST300 but could be a Powerglide. I know they worked okay behind a 389, but it's the original trans with 180,000 miles on it and I'd much rather have a 200r4 or TH350.

And finally the rear end is most likely a pegleg, and I want posi. Would a later (68-72) rear end work?
 

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The 455 will go right in. You should use a TH400 trans. The 68 rear is 1 inch wider than the 66, but will fit. You MAY need diffrent coils for the rear swap. Eric
 
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