You're going to have to do some splicing if you want to keep your original 73 harness. As you know, the bulb sockets are on your 73 harness but the 70-71 side markers have that little male/female side by side connector. So you either have to get a 70-71 harness or put some of those ends on your harness.
I would crimp, solder, then heatshrink the spliced wires before wrapping them up.
You can get that connector already molded in rubber at some electronics stores. This way your splice is a few inches away.