I was going to run the black and yellow power supply wire from the original fuse box to the pos terminal on the new fuse block.. it comes from the alternator for all the constant hot circuits
Correct. Should work fine.
Note one additional thing in your diagram though. See the shaded thick lines connecting some of the fuses? Those indicate buss-bars interconnecting those particular circuits to each other. So the Black w/yellow wire powers every fuse on that particular buss-bar, while the Black w/green wire powers the fused circuits on the other buss-bar.
Effectively then, circuit 2a powers fused circuits 1 and 2, in addition to the two blank spots you can see next to them.
Black w/yellow #6 more obviously feeds #4 and #5.
...but the black and green wire you’re referring too leave the ignition post and goes to the 14 Amp fuse in the original fuse box. So I can Cut this black and green wire off of the original fuse box and connected it to the other positive terminal on the split bus fuse block?
Correct.
And It will supply power to those fuses once the ignition is turned on or is in the ACC position?
Correct.
Also can I place a new fuse in the new fuse block for the wires that share the same fuse in the original fuse box? Wires 2, 2a and wires 3 and 3a?
In theory you do not need a fuse on 2a (Black w/green) because there is none there from the factory, and it does not have any other unprotected loads attached to it to protect. Other than the wire itself.
In practice of course, it's not a bad idea to have one. Up to you, but in my case I usually leave those unprotected, but do my durndest to make sure that the wire itself is protected.
The Blue w/red wire should definitely have it's own new 2a fuse in the new panel. But as mentioned, the power comes from a source other than the Black w/yellow wire, because it's controlled by the rheostat/dimmer thingy in the headlight switch.
In most Early Broncos, that circuit only controls the instrument panel illumination lamps. In '73 and later models this included the dash "light bars" indicating the other control knobs.
Some EB's were optioned with a dome lamp (in the electric wiper motor cover) and/or a map light (under the dash) that were only lit when the headlight switch rheostat was turned all the way to the click.
Broncobowsher is correct in general terms with regard to probably most Broncos. But if your overall wiring is in good shape and is lacking the usual PO hacks, nothing wrong with continuing to use it. I could do it on my '71 because the wires are practically perfect. There are however, way too many Broncos who's time has come to retire the original wiring.
So quite frankly, if any of your wires are feeling old and tired, dried out or worse still, cracking up, or a PO or three have had their way with it, it's time to replace the entire harness.
Good luck.
Paul
with 6 Constant hot circuits and 6 circuits which are only hot when the ignition is on, so the dome light and the courtesy lights will have power running to them when the ignition is off. Is this true? Or can I use the standard fuse block?
