magnum build

Got my magnum engine stripped down. Its got a 120,000 miles on it. From what I can tell the cylinder walls loom good. But any info on what I should be looking for please let me know. Going to put new rings and bearings in it next time I have a chance to work on it.

The pistons have a 4 stamped on it. I'm assuming that means 4 inch bore. I'm assuming its a stock bore but want to be 100 percent certain before getting rings.

Leaning towards the eq heads. Not sure on the valve size. I'm guessing for a 400 horsepower build the smaller 1.9 valve size would be better. But I don't know?

Any help on this stuff greatly appreciated. This is my first build. I'm a rookie. Thankfully I got some help from my dad on things. But neither him nor I know much about what parts work best together for my goal of 400 horsepower. Or I should say anywhere from 380 horsepower and up is good. But I imagine to much over 400 would make the car less driver friendly on the road or cost more money than I want to spend.

I'm putting it in a 69 valiant. Its light and I think even 400 horsepower would make the thing fast enough for me. The goal at the drags is in the 12 second range.

Thanks!