The easiest way to 300 to 350 horsepower is WWW.GMPARTSDEPOT.COM,
look at the crate motors - brand new 350's, 290 horse for $1599.00, 330 horse for $2398.00, or a factory stock class circle track race motor for $2895.00 - these are factory new engines from GM and will hold together - they are priced comparable to a performance rebuilt, but you get standard bores and all new components! BTW I am running a crate HT454 (L-29 Engine) with a cam change - replaced the HT cam with the ZZ502 cam - and couldn't be more satisfied - it was lots cheaper than a rebuild was going to run, and got a roller cam engine with VORTEC heads, tremendous torque that will run on premium unleaded without having to buy race fuel, and has been giving excellent service in my jet boat (It is not exactly living a pampered life!) I topped mine off with a Holley Street Dominator manifold and Barry Grant Speed Demon carb - and Mallory digital distributor and ignition - I can't say enough good about it! It kicks butt - lots more power than the older gen IV big block we ran previously, with less duration amd lift but a lot fatter cam lobe with the roller. These crate motors are engineered packages - developed by automotive engineers with lots of experience and expensive dynomometers at their disposal - there is a lot to be said for knowledge and experience!
Just my 2 cents worth -