[HardcoreZ28]

Going from a 305 to a 350 TPI is no problem with emissions as long as all of your sensors and pollution devices are still in tact. I am running a 383 in my original 305 car with extensive modifications and the car runs cleaner than the day it was new.
I understand everyones argument about crate motors but I'd had good luck so far with my GM ones. If you want an easy way to make 350-400 horsepower I say get the GM ZZ383 Long block, a Holley Stealth Ram setup, and a custom PROM. Swap all of your sensors and accessories over to it and that's about all there is to that. You should have no problem passing emissions and the car will be a great street performer.
Any other questions feel free to PM me....I did my swap about 5 years ago but still remember most of the info. I believe the only things you have to change between the two motors are the knock sensor and the knock sensor module. Oh and the injectors of course....need to flow enough fuel for your power level.