I know there's lots of praise for LS7 and LS9 engines, but I too would seek out an LS3 equipped car. That's the "sleeper" engine. It makes great power, but GM left lots on the table. Mods aren't cheap like for Ford 5.0s, but not German eye-watering either.
LS3 crate engines are a popular transplant into some pretty potent track machines (Miata, RX-7, etc) and have proven durable, even when making well over 500hp.