73 318 will J heads work???

The 73 models were probably the low point in terms of compression ratio. The J heads, with their larger volume, would make the situation worse. It can be helped with 67-72 stock pistons. These were flat tops with 4 valve reliefs on them. They also had the highest deck height of the 318 pistons.

I built a 318 like this with a P4452761 Purple shaft & J heads. It was a little down on power until you got it over 2500 rpm. Using a similar cam in a similar 318, 302 heads, and the same intake/carb combination, throttle response is considerably better. For a mostly street engine, I prefer the 302 head combination. I agree with those who recommend that if you want to use the J heads, the compression ratio needs to be bumped for a normally aspirated engine.