[bwdz]

1971 the 302 put out 210 hp in the Mustang (if that is where your engine is from, it put out less in other cars) That 210 hp was rated the old way meaning not SAE so to compare it to the way engines are rated today it was about a 160hp engine. By changing to a 4 barrel carburetor and aluminum intake and you probably have better exhaust than factory you are probably in the low 200hp range. I used to dyno lots of Mustangs back in the 90s and a "hot 302" with good heads and a nice roller cam got around the 300hp mark and more if it had high compression ported aftermarket heads etc..
What I am saying is that you are around the 200hp mark, if you start making 300 you will feel like you just turned your car into a Ferrari. Get the AFR heads if you want but I would just build a new engine in the garage using a roller block from a later model and not use that old block to build a new engine. This way when the other engine is complete just sell your old one and install the new one. Seeing that you mention you are in Spain it is probably not as easy to find a roller block as they literally lay around in garbage piles here in the states but I would not throw those great heads and stroker kit on that old block as you will only spend a fraction of what you spend on those other parts on the newer better block.