With the new clutch technology with the Soft-Lok's and two sets of springs with different rates and some with two sets of fingers, clutch size is no longer the determining factor... As Nitro said, the contact area is what counts, not the od of the pressure plate. I use a 10.5" McCleod diaphragm that hooks harder and lasts longer then any 11.5 or 12 inch Borg & Beck style 3 armed overweight clutch/flywheel setup that ever came out of the factory.... When I step up the power (over 500hp), it'll probably go down to a 6.5" double disc.....

For the 454, having the external balance flywheel is going to be the main concern. If the flywheel isn't drilled for a 10.5 or 11 inch clutch, it's a fairly simple operation for a machine shop.....