Junior, since you brought it up, what is the relative crankcase ratio of the early twin bottom end compared to and SRX, you made the statement, therefore you must have checked both, as well as, the piston surface area between a 74.5mm piston and a 69mm piston with the same dome arc, the larger piston has less surface area? The 74.5 piston would have to flat for that to be true, or the dome on the 69mm piston would have to be 2mm higher than it is, and it isn't.
Anyway, who cares, I built it, it's a cool sled, and it looks like it came that way. Maybe thats why it was in the Yamaha booth at the snow show in 96', (note the pic)they thought it was a nice piece too.