You're doing a fully custom setup, so only you can answer those questions. If you mount the motor correctly you shouldn't have to recess the firewall.

My dad has a 454 in his '52 with the stock firewall, so your LS should fit just fine.


1946 GMC Pickup - S-10 Frame, 455 Buick, TH400, original patina.

My 46 GMC on Photobucket