The seat-T is from V-Twin. The throat is too fat to allow the seat spring to seat. It takes about 20 or thirty grind and fit sessions, but you can make it fit and work like it should. The holes in the spring steel plate need hogging out to match the seat studs.
I've used a Foredom cable grinder a lot in kit building.
http://www.blackstoneind.com/foundation ... &category=
An 1/8" diamond-dust cylinder burr expanded the slot holes in the T. You can find burrs in an assorted set cheap from Harbor Freight.