Chet, I think there's at least one digit missing == perhaps an initial "3" or "4" --as well as a final "L", as I'm pretty sure the 1969 US-model Spitfire began around FDU 31254 L. The 1968 and earlier Mk3 Spitfires in the US. were FD (no "U").
As for the engine, original specification would have been dual SU HS2 carburetors (not Stromberg); the single Stromberg CD150 came in with the US 1970 model year. There's a good chance your engine is not the original, but the engine serial number could confirm that.
Joe Curry's International Triumph Spitfire Database has a section for the Mk3 at:
http://members.cox.net/spitlist/Mk3.htm[Interestingly enough, there is a "FD 39941 L" (no "U") listed, but the color stated is yellow. It could be a repaint, but most likely it's not your car.] Is there a body number on the car? It would be on a painted plate below the commission plate, and it would read xxxxx FC.