Well first thing that would help out a lot is what are your cam specs and there are a lot of guys on here who know there stuff on chevy factory heads.
If your wanting to save money and I know your looking towards that you could just keep it the 350 cubic inch for now and if your able to get a set of vortec heads for dirt cheap paired up with your cam and if its not too big on the lift specs and your can get the proper springs to be put on your vortec heads you could make 300 very easy if not more.
I just had my flattop 350 topped out with a set of rhs pro action 180cc heads and a zz4 roller cam and it runs very strong and still drives through town with a 350 turbo and 3.42 gears and my s10 is a 96.
I am sure someone on here will post some way better info that I just did. Vortec heads are very good and easy to make 300-350 horsepower no problem with a mild cam. Only thing is the springs to be matched to your cam.