Comp cams xe268h for 318

In theory that is correct. But just try to find a cheap hi-compression piston at factory weight. I wouldn't spend a lotta time looking; I'd just order the KB107s.
Maybe someone on FABO has done the homework.
In any case the 107s are quite light at 502 grams,versus the factory slugs. They mightabin 545 or 592,I can't recall. The balance shop will need all your reciprocating parts including the balancer and either the TC or an aftermarket flexplate; to figure out a new external balance factor and set it up. That is the cheapest.The factor goes onto the TC or F-plate, the balancer is usually not touched. Bring them the parts they ask for, and they will give you your options.