My 54 has a 327 in it. You will definately need to chop those fire wall braces.
My car has been clipped and a 78' Camaro subframe welded in place. I would recommend a narrower MonteCarlo unit though. You'll get into trouble with axle width and tire clearance if you lower it too much. There is also lots of room for an HEI distibutor. I can easily pull mine in and out with no fire wall clearance problems. I love the power steering, disc brakes, & modern suspesion setup. I've got a Nova rear axel and standard drive shaft.
One big problem with clipping a car is that you now have to fabricate mounting brakets for radiators, bumpers, fenders, batteries...
You can do alot of these updates to your original frame via kits. But they can get pricy.