When buying new, you want better than the basic crate 350 or the 350/290 hp (same low compression engine). Its exactly the same as a mid 70's smogger 350, with inefficient heads, low compression, low lift flat tappet cam, and two piece rear seal.
If you move up to the L31 engine you get Vortec heads, roller cam (191/196 @ .050) and a 1 piece rear main seal. However, you do lose the ability to use a mechanical fuel pump. If you buy that L31 crate you will have the option of later tweaking the heads to allow more lift, and then add a higher performance roller cam that will give you much more horsepower.
Look for the engine masters episode on youtube where they took the basic crate 350 and dyno'd it at about 240 horsepower. Then they bolted on Vortec heads and a mid range flat tappet cam and brought it up to about 350 horsepower.
Blueprint engines also has a crate 350 with Vortec heads and higher performance roller cam (220/224 @ .050) that makes 373 HP, and its priced at $2,599. Its more than the L31 crate, but it would give you quite a bit more horsepower right from the start.
Bruce