5.4lt boss can definitely be fitted. Yep, the same one used in the BA Falcon etc.
It's a very heavy lump of an engine though with it's cast iron block. Much, much heavier than alloy 4.6lt engine. 4.6lt is a much better looking engine out of the box as well.
My recommendation would be to stick with 4.6lt, if you want more poke, then go with Supercharged version. /ubbthreads/images/graemlins/grin.gif The 5.4lt is larger and heavier, so it is not as straightforward an improvement as you would have expected.
But at least you have the option and a few people seem to prefer the larger 5.4lt Boss engine. Supercharged you would have a very similiar engine to the Ford GT. /ubbthreads/images/graemlins/smile.gif