Combo opinions...

A 73 318 with stock pistons won't have much more than 8.0:1 as delivered. You're driving a hard bargain IMO to get 300hp from that. Not impossible, but a taller order. I'm not a fan if milling the heads beyond .030 or so. You have to mill the intake side too, and beyond .030 or so you may have to mill the intake's flanges to fit it. Plus it will require checking and getting new pushrods. The extra milling and pushrods mean more cash. If it were up to me I'd say build it with stock pistons, stick that XE268 in it, and don't worry about what the resulting hp will be. My impression is it will end up a little shy of that 300 number, but who cares. It will sound close and you can tell them what ever you want...lol. On the valve sizes. There's really no "sides" as port flow and valve size go hand in hand and ideas of how they work are like assholes. Everyone has one. Mine opinion is go just large enough to be able to put the seats out at the edge of the chamber with enough flow to not be the choke point and you're fine. Personally I'm fairly good at feeling performance and I know I can't drive a street car that's properly tuned and in good order and tell you what's in the heads for intake valves. So in that application really what difference do they make?