HP gains over stock heads?
 

I've got a 355 in my 71 monte with shorty headers, comp thumpr cam (the small one), performer rpm intake and a holley street avenger 670.

What sort of power boost would I see with some good heads?

You are going to have to be more specific on your combination. Like: compression,cam specs etc. Gains are based on parts working as a system together.

You have a myriad of choices based on a Chevy small block heads that have the potential for power/torque. Think of a combination then we can work from there. ---Good Luck.

Not apples to apples but here are some real world numbers I experienced
9.8 to 1 383 mild street strip car with mild ported 882's consistant 8.20's in the 1/8th. No other changes than a set of Iron Eagle 200cc heads out of the box and the car woke up instantly to 7.60's best thing I even did to the car.

It all depends.
You spend the money on a good set of heads and you "may" get an extra 30-40 horses.
It all depends on the heads you buy and how they work in conjunction with your current set up.:thumbsup: