Best bang is what you have, vs. what you can spend. C-90's are great, but tough to find without spending big $$$; O-200 are great because you can find them anywhere, but the cam doesn't make power as good. Super-85's combine the best of both; power of the C-90, and availability of the O-200.
Fairly certain you have to use a C-85 case; my copy of the STC only mentions C-85's NOT the C-75. I'm sure it can be done, but to make it legal will require a 337/field approval. Same thing with using C-90 parts; they're not listed in the STC.
The original STC had you use a C-85 core, with O-200 crank, rods and pistons. But, if you have a good O-200 core, you can combine a couple different STC's from Don Swords and use an O-200 case, crank, rods, pistons, gears, and cylinders with a C-85 cam and achieve the same thing. Basically an O-200 with an 85 cam. All done legally.
If you're serious; best thing to do is call Don Swords with specific questions about what parts you have on hand. He can tell you what will work and what you need to finish.
Good luck; it's a sweet-heart of a motor.
nkh