Paul,
Good idea asking about a "spare".
In case you want a new one, check out Chevy Duty:
Part # 79-204 "Complete steel and rubber bracket assembly for transmission tailshaft. 1947-1954(sic)"
http://www.chevyduty.com/frame-store.html Neither American Classic Truck Parts nor Jim Carter appears to have the rear transmission mount.
Your frame cross-member for this mount must still be there - right (I have heard of people taking this crossmember out - don't know if that is a good idea, but it happens)?
Good luck,
Tim
