I have a B670, best I can do is show some pics of the mounting scheme. I would think you could make a cardboard template of the mounting plates, as I remember they were by no means a precision fit. The only purpose the plates serve is to provide a pocket for the round frame crossmember to sit in. (loosely, no firm attachment) The B670 was intended to attach to the B219 FEL subframe, and that is the only place it is actually bolted to the tractor, the crossmember "floats" in it's notch. I guess the first question to ask would be "Do you have the B219 loader & subframe?"
Also pertaining to your gasket for a template idea, (good one!) try Coleman Equipment, they have had virtually every part I've ever needed for mine.
http://www.colemanequip.com/Kubota_Parts.asp
1st pic shows the plates (in black) with hole for the 3pt bar, and shows the round crossmember in it's notch.
2nd shows the B219 subframe (upper horizontal orange bar), the B670 frame (lower horizontal bar) and the bracket that bolts them together.
3rd shows the plates & the crossmember in it's notch, free floating.