yeah you can put either a 4.6 or a 5.0 in there... it'll be a PITA, and you'll probably need a doner 5.0 or 4.6 to make it easier. your best bet is to just buy another GT.
Well I pulled my engine out expecting to throw in some forged pistons, new bearings and rings, and here I am two weeks later dropping off a 351W at the machine shop.
But again, to keep this on topic as to not piss off anyone, yes a V8 will fit.