I would build a pontiac for the bird...a very mild 428 or 455 could get you in the low 12's and still be for the most part stock.I only run buick pontiac and old's, once you taste 510 foot lbs of tq you will never want a small block again. With a 455 in that car you could still have nice gears for highway use and the 455 wouldnt know the diffence, where as a smallblock NEEDS gears to move a car that heavy.
I have a 1968 428 in my 67 firebird,3.55 gears,headers msd ignition and a holley 850 sitting on top of performer rpm intake and a ram air IV cam. I drive it everywhere and still run 11.93 @ 116mph