I've seen big block Chevys in these trucks, but don't know that I've ever seen a big Pontiac in one. I'm sure it can be done, and if you are using the Firebird clip some of the problems (like steering interference) will be taken care of as part of the clip project.
Firewall and floor really depend on where you mount the motor. As you clip the truck you'll be deciding how much to lower the truck, and that will affect your options for mounting the motor.
I've always thought it would be fun to get some of the GMC valve covers used on '55 to '59 GMC V-8 powered trucks because many of those motors were Pontiacs. I believe the GMC logo is actually a decal, not embossed, but I understand that though it may take two gaskets to ensure enough room rockers to move, it seems like a fun way to make that big Pontiac look like it grew there under that GMC hood.
Good Guys Puyallip in next weekend. There is a guy who usually shows his '54 (actually its a '55 1st, but he calls it a '54) GMC with an Olds clip and a 454 Chevy under the hood. If you make the trip you might want to look for it.
