Well, people have done v6 to v8 engine swaps plenty of times. Your resources, mechanical abilities, and financial support determine how easy this can be done.
Typically here, most will say that a 99-04 v8 donor car is your best bet. This way you get everything you need: complete engine, ecu, wiring, 8.8 rearend, etc.
If you can do 99.9% of the labor involved, then I'd say do the swap.
But if that's not the case, I'd personally tell you to either sell your current v6 and buy the v8 (there are plenty of decent v8's out there for cheap), or save up and put on a power adder along with some supporting mods to get your v6 moving better.
In all reality, the engine swap is more expensive and could be more of a hassle than just selling your v6 and buying a v8. I found an 03 GT parts car for 3k obo on craigslist. The engine/tranny had 55k miles on it. So your already at ~2.5 to 3k for the donor car, plus the labor to have the swap done. But in the end it's up to you. That's my .02