Interesting that you should mention that...

I don't know how you would bolt them together, nor transfer the marking points. I guess if you made a mandrel of some kind that fitted between the flywheels you could get them on the same axis.

Ultimately I think I might make a drilling jig based on this kind of idea, then I could repeat the process on both sizes of flywheel whenever I need to.

There'd be a lot of machining involved but I can take care of that.

Speaking of the flywheel sizes, here are a couple of 130-tooth flywheels I photographed as well: One is untouched (as far as 360 external balance is concerned) and the other is the one I had drilled in Spokane when I bought my van there. Unfortunately, that drilling isn't repeatable because the bottoms of the holes are round. Anyway, notice these have the different pressure plate bolt locations:


Edit: I've changed the second pic in this post as I realised it was a Slant flywheel. This one is a 318 flywheel, but maybe a 340 with the scallops and the ⅞" deep drillings in it. I have a couple with these deep drillings, the other having four of these holes (approximately ⅝" diameter) rather than the five in this one.