You can't install the 290 hp crate engine from GM because it uses non Vortec heads on it. Vortecs have a specific intake bolt pattern uncommon from any other small block. That and reall that engine is a 100% backwards step being it's a pre 86 leak prone flat tappet cam small block.
I think the lowest rated Vortec headed GM crate is the 330 hp 350 HO. I wouldn't buy it either as it's nothing more than the L31 crate engine with chrome valve covers and a flat tappet cam instead of the roller installed in the factory replacement engines. If you took an L31 Vortec 350 crate and dynoed it in the same fashion as these crates it works out to about 320 hp. The service replacements use the SAE net rating system whereas the GM performance crates are basically rated Gross Horsepower (like how all engines used to be rated before 1972). SAE means flywheel horsepower but with the full exhaust, tuning, and all accessories the engine would have when installed in the vehicle.
Just buy an L31 replacement engine and if you want more out of it, do a mild cam.
