If you find a 68-70 teener, a lot of times the castings have thick enough walls that you can bore it out for standard bore 340 pistons. Being that a 318 and 340 have the same stroke crank etc, you can build a really nice one.
Ya,ya,ya no replacement for displacement. I get that. But the reality is as light as these cars are, for all around street use, and weekend funning around, how much HP do you realistically need? I'd say a decently geared car, with 325hp teener would be a lot of fun, not too expensive to build, and still be driveable.
What I mean by driveable is it doesnt idle like a coffee can full of rocks. You dont have to beat it with a whip to get it out of the garage. We all like power, but at what point does it become something that you wouldent want to live with every day.
I hear of guys with nicely tuned and running fairly quick teeners and stocker slants that pull em and build 450-500hp strokers for them, then they dont enjoy the car as much anymore. Dont drive it as much anymore.
Everything is a trade off.