Seems like with what you have there, depending on valve sizing, you could get into the 300 + HP range with the proper cam, intake, carburation, exhaust/headers, compression, etc...
I have ported 289 heads on my 302, 1.94"/1.60" valves, 0.560"/0.560" lift, 294 duration, solid lifter cam, single plane intake manifold, Holley 780 cfm "racing" carb, etc... etc... and am making about 400 HP. Will be taking to the drag strip in the next week or two to verify with some ET's. Not very user friendly out on the street, but makes LOTS of power.... fun, fun, fun, til' your daddy takes the T Bird (Cougar) away.
The question of "How much horsepower" is much more a function of $$$$ than anything else.
Ron.