Sorry I haven't been on the forum been busy with the fourth stuff, parade barbeque,etc.
No I wouldn't recommend tieing into the battery positive post but you don't have an easy way to get to the main junction. It's wrapped in the harness just below the left headlight. the factory soldered all the wires together because it was simple and cheap.
Here is a picture of mine after I unwrapped it.
and a little closer.
This is a diagram of the factory wiring from the positive post.
It shows how your wires from the battery,alternator, and regulator, join, and the red power that goes into the firewall block to feed the cab.
These red wires are connected together directly so you can see how splicing the headlight relay power wire into any of them would power the relays.
The alternator post or the no. 3 red wire on the external regulator would be easy places to connect if you didn't want to untape the harness.
Most of the diagrams for the headlight relays call for a 12 gauge wire with a 30 amp fuse to the no.30 terminal on the two relays. A downside with only running 1 wire is that if the fuse blows neither of the relays will work and you won't have any lights. If you run two wires, one to each relay with two fuses then if one blows you will still have the dims, or the brights to get you home.
Here are two diagrams for the headlight relay mods they are slightly different.
The second shows the wire gauge and 2 20 amp fuses and both of them show wiring from the battery positive or any 12 volt source.