Jon,

1. It looks like the 751 should be about 55 ~ 60 mm from the mounting face to the center line of the tube. (Normal OS muffler distance fromt the exhaust face to the center line of the muffler).

Distance between the center of muffler mounting screws on the engine is around 43 mm. If you look at the gasket and draw a line that can be rotated to 90 degrees, you should be able to guess the distance from the mounting face to the center of tube.

750 is pretty long for side mounted engine and I think it is to clear the fuse bottom.

2. Mounting bolts for 61FX center to center on rectangular 4 bolt pattern is 52mm (width) x 25 mm (mounting flange).

I believe SF and FX are still interchangable for mounting.

3. Outside dimensions are same for 650 & 668, but I am not sure the internal baffle differences. Maybe they made 668 a little quieter or more power boost?

Why not use 700?

Hope this helps,