I put a newer R-Spec seamless tank on my C-Spec. The only difference was a shorter bolt and plain washer is used to hold the rear brace instead of the long bolt and collar. The seamless tank has the collar as part of the tank and it's threaded for a short bolt. Old tank bolt looks like M6x30 and new bolt, M6x12.
Old tank parts, 90387-06177-00 (collar) and 95812-06030-00 (flange bolt). Current tank parts, 90201-06110-00 (plate washer) and 92012-06012-00 (button head bolt).