I am sorry but I am with Buzzard Bait on this. Your chances of getting a nitro powered RC conversion of a Guillow 400 P 51 off the ground and back in one piece are virtually zero. Even a very experienced builder and flyer will find it difficult.
For a 0.49 [ presumably a Cox of some variety I would look for a high wing model around 33 to 36 inch span and RESIST THE TEMPTATION TO BEEF IT UP Light models fly, heavy ones crash.
As for a throttle you can find several alternatives on flea bay. A reed valve Cox does not throttle well but you can get a sort of half power setting for a while.