In terms of tire size, I've been playing with transmission gear ratios, ring and pinion ratios, tire sizes, engine rpm's, and target speeds on a couple of spreadsheets.
I have 2 combinations that look promising to me, both of which require shorter tires for the speeds I'm looking to run and the favorable rpm window for the engine. I'm afraid that if I went with the larger tires, the engine would "lug", and not be able to pull through the rpm range.
My thinking is to have 2 sets of matched pairs of tires, with slightly different diameters (front & back). Then, a quick change of front tires to rear, and rear tires to front, functions like a quick-change set of gears.
The seat is an UltraShield. This one features what they call their "Bonneville" headrest which is designed specifically to meet the lateral head restraint rules of the SCTA.