It wouldn't let me add more by editting so .....
The above method of transfering hole is excellent. However, in my best been there, screwed this up voice, mark and drill the first hole, set your transfer point for the hole on the other frame rail, and bolt the first one down. This will ensure the body hasn't shifted while transfering holes. Repete this process with each hole. For those who have a bare frame, use 6 - 3/8" bolts, lock nuts and a fender washer or equivilent in the body and 3 in the box, one on each side rail and one centered on the rear cross member. You did glass a floor in the box right?
Ron