It's going to be tough. There are two issues: the obvious difference in the horizontal and vertical resolution of the comps, and the other is pixel aspect ratio. If your 720x480 comp is widescreen, the pixel aspect ratio is 1.33; if it's a 4x3 comp, the pixel aspect ratio is 0.9... but in the 1280x720 comp, it's a 1.0 (square) pixel aspect ratio.
Try working in the 1280x720 comp. Get the numbers to match up. When done, nest the larger comp inside the smaller one, and scale the larger one to fit.
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA