Magnum 360 heads "best" for my build?

3.23 gear at the least.jmo

polish anything that might be crack prone=between valves around chamber, yes they crack there too, and you'll be good.jmo
Especially in the bowl between the valves, I've seen them crack from the seat down almost to the bowl roof.

you should be good though with what you are doing as long as you have enough capacity in the rad.

Good advice on the polish, easy enough to do while the rest of the build is simmering on low ;)

I will probably start out with the current converter and gears just to see how it runs, feels, launches, etc. Then I'll make some decisions and upgrade the gears and the converter - might as well get the experience while I can.

On second thought, best time to change the converter is when I swap the 360 in place of the 318, no reason to pull the trans twice

Honestly 3.2X gearing is a compromise, I do really prefer to drop in 3.55s but not on the fwy at 80mph with 1:1 top trans ratio.