I wouldn't be afraid to put a 4 inch crank in a 318 with the heads you have. It will just be a torque animal, and make more hp than the 318 for sure despite the poor heads. I had a set of 906 BB iron heads on a 535 (!) and it went low tens with nothing more than 4.10 gears turning 33 inch tall slicks in a 3400 lb car.
With the 4 inch crank, you can run a rather tight converter and still get decent performance. Not so with a 318,,,,,,, The cam I ran (mid sized muthr thumpr) will like the 4 inch crank a lot better too. When you want to add some power, go with the EQ heads and copy my basic build, and you should get about 450 hp!