I built an LT-40 from a kit and fitted it with an O.S. 46LA also. The motor mount supplied in the kit was ample long enough to get the prop washer 4 3/8" away from the firewall, so my guess is that they've supplied you with a motor mount that's a little too short - maybe one from the LT-25 kit. Drop SIG a note explaining your problem and I'm sure they'll supply you with the correct part.

Cheers,
Neil.