Building the engine to a 408 stroker will not physically alter the the external dimensions of the engine. The 6 Cylinder engine compartments are the same physical size as the V8, just a motor mount change, but you will want ot look at suspension, brake and cooling system upgrades along with a possible rear end upgrade.

The 351 based engine should slip right in, but you will definately want to install a set of full legnth headers to get the maximum benefit from what ever engine combination you end up using.