318 roller motor

Zinc alone isn't a reason.
The real reason would be that roller cams make more power, one reason is they open faster and stay at max lift longer allowing your to really flow.

The stock spec'd roller in the 318 sux, But you can have reground to a better profile and then buy a snout extension if need be for the fuel pump and your set.

If you wanna just throw a flat tap in there it slides right in and only really needs some regular old hyd push rods, did the same thing with my 84' 318 roller motor+ 340 cam and 67' 318 closed chambered heads.