Cheapest way to make a 350 breathe fire is... SDPC2000.com modified vortec heads, 1.6 roller rockers, comp XE 268-274 cam, and a decent induction system. You are looking at over 400 hp.

For a more fuel injection friendly engine making more power. Pro Lightning heads from shaver racing (about $700 for an assembled pair) LT4 hot cam, and 1.6 roller rockers. This enigne will eat LS1's for lunch.