AS FAR AS I KNOW, THE BOLT PATTERN ON A BIG, 14in DIAMETER FLYWHEEL, REGARDLESS OF WHETHER IT CAME OFF OF A 350, 427 OR 454, CAR OR TRUCK, WILL ACCEPT EITHER AN 11 OR 12 PRESSURE PLATE. I once had a 12in pressure plate in the shop, but I must have used it for a core trade-in on an 11in pressure plate. Although, I do still have the 12in clutch disc. THE 12in DISC EASILY FITS WITHIN THE BOLT PATTERN FOR AN 11in PRESSURE PLATE. As I recall, the bolt holes on the 12in truck pressure plate were tucked in real tight to the pressure plate. Therefore, as I said, I'm sure both an 11 and 12in pressure plate use the same bolt holes on a big, 14in flywheel. But, for a car application, I just don't think you need the bigger, heavier 12in clutch. That's an awful lot of mass to spin at the rpm of a medium or high perf engine. Truck motors don't spin that fast.
And also, I DO NOT BELIEVE anything bigger than a 14in flywheel can fit within the confines of the truck bell housing. So, if you have the big truck flywheel, use it with an 11in clutch. Remember, a 454 flywheel is EXACTLY the same size as a big flywheel used on a 350-396-427, but the 454 flywheel is all by itself because of the required extra weight on one side for balance purposes (if you are building a SB400 with a 4sp, use a 454 flywheel on the SB400 and just have it balanced with the engine, because flywheels for SB400s with a manual tranny are super scarce).
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Tom Parsons
