Which heads?

don't retrofit. your return on investment will be bad. unless you patch up the comp ratio you are gaining little or nothing. gain some flow but lose some hp due to compression loss.

your best investment would be to drive it like it is. get a 360 somewhere. tear it down in your spare time build it right. get 9.5:1 and the 308 heads or better even.

that way you can drive now, will see good improvements from your money. everyone is happy. after the 360 is done and in, you can save the 318 for a later project or sell it. >>> best bang for the money

I've put 360 heads on a 318 before and the improvement is minimal.