Thoughts on early vs. late Slants.
Different pilot size, I thought?
OK, if you're using the later flywheel with the early engine, I believe you're right.
However, and I've not tried this with the O/D trans, but I used my original 65 170 3-speed flywheel with an early 70s slant six and a 65 4-speed bellhousing and trans. All I had to do was machine the center opening in the 65 flywheel to fit the later crank's register.
If you can keep your original 66 flywheel, it of course won't need to be machined for a 66 engine. But I don't know if your O/D trans bellhousing takes the same size clutch as what you already have.
Apologize for editing this a couple of times.