well, unported afr's i think would grab you an extra 20-30 hp i would guess, possibly even more. I think i've heard of gains around 40 hp. Correct me if im wrong.
My opinion, go for nice set of heads and a good cam. Afr's and a good cam, you can expect a good 50-60hp.
What are you making now? A good set of aluminum heads with matching cam will make 290-325 at the wheels, depending on the supporting goodies and the tune.
P heads can make really good power considering the price. There was a dude on corral with P heads, afm b-25 cam and a cobra or tfs street intake (cant remember) and made 299rwhp. Under the curve power was outright awesome. More importantly, it ran the # at the track. It went high 7's @ 86 in the 1/8. Thats about what your typical afr, tw or edelbrock head will/could do on stock short 302's. Most P headed combo's you see make 260ish but thats with junk cams. I dont think a $1300 set of afr's will be a cost effective mod for you. You might see 20hp with them. I would change out that B303 cam, add longtubes and some nitrous.
My friend just made 323hp/346tq on his 306 with a set of new edelbrock heads.