Is it safe to weld a patch into a cracked frame on a 1979 Ford truck?

If it is done properly,...yes.

The only way to repair a split or weak frame is to fish plate it. What this is, is a series of 1/4 in. steel plates 2 inches wide and 1/2 inch shorter than the height of the frame rail. The first plate is arc welded on the frame with a continuous bead on all 4 sides a minimum of 2 inches forward of the damaged area. The second plate is then welded over the first plate covering one inch of the first plate.The same for each 2 inch plate until the damaged area has been passed by 2 inches. The reason we call it fish plating is that when it is done, the repair area should look like a row of fish scales.