There's a big difference between 350hp and 500hp you need to narrow it down a bit like pick a number + or - 25hp, 350-400hp ain't too hard. What the application? That's a better place to start than some vague power number.
Also check your local pick n pull prices. A long block here cost $130+ $30 core, short block is $90+ $20 core. I don't know your prices but its worth it to go hunt down a roller block.
