I would just go with the Goodwrench 195hp 350. It is in all essence an L48 except with 4 bolt mains right down to the camshaft and heads. I've seen this engine built up to over 400hp gross so lots of potential in it. The GM Performance 290hp you are looking at is the same engine but with the L82 cam for a lot more money. Pretty expensive camshaft.
http://www.summitracing.com/int/parts/nal-10067353/overview/
http://www.summitracing.com/int/parts/nal-12499529/overview/
Good story here on the Goodwrench 350:
http://www.chevyhiperformance.com/techarticles/46320_gm_350_crate_engine_build/
http://www.chevyhiperformance.com/techarticles/46326_gm_350_crate_engine_build_ii/
http://www.chevyhiperformance.com/techarticles/46359_gm_350_crate_engine_build_iii/
http://www.chevyhiperformance.com/techarticles/46364_gm_350_crate_engine_build_iv/
http://www.chevyhiperformance.com/techarticles/46370_gm_350_crate_engine_build_v/
http://www.chevyhiperformance.com/techarticles/46438_gm_350_crate_engine_build_vi/
http://www.chevyhiperformance.com/techarticles/46461_gm_350_crate_engine_build_vii/
http://www.chevyhiperformance.com/techarticles/46467_gm_350_crate_engine_build_viii/
http://www.summitracing.com/int/parts/nal-12499529/overview/