I don't get how you are gonna manage to pull this off I've done a quick research and unless I'm looking at the wrong engines they go for about $30,000 in your class and that's Chev probably can add another $10,000 for dodge. It's not that 650 hp is exactly a hard number to hit especially on methanol but the reason these engines cost so much is there built for endurance and even with all the best parts they only last 15 or so races.
I'm all for the under dog, I agree the 340 would be the better engine to start with but 318 is what we got so let's move on you've pick your engine so valve shrouding is a non issue it's all about trying to make it work now.
The biggest hurdle now is cylinder heads what's the plan there ? Probably your best bet there is to find 2nd hand ported out W2's
