adding headlight relays in a '72 (with existing headlight flasher relay)
 

Alright, I have researched this exhaustively and have determined I'm not smart enough to figure this out on my own. What I have figured out:

1) Sucro relay kit is not plug-and-play for pre-'74 cars
2) '72 (and I'm sure other early cars) has an existing "headlight flasher relay" indicated at [50] on the wiring diagrams
3) high and low beam outputs come from the headlight flasher relay [50] respectively at relay outputs 87a and 87
4) high relay output is bridged at Fuse Block II at [1] and [2] and respectively feeds left and right headlights
5) low relay output is bridged at Fuse Block II at [3] and [4] and respectively feeds left and right headlights
6) my '72T manual indicates that Fuse Block II [1-4] should be 8A fuses, but I have a random mixture of 8A and 25A fuses in those locations

What I don't know:
1) do I add the relays "downstream" from headlight flasher relay [50] and Fuse Box II?
2) do I fuse Fuse Box II [1-4] with 8A fuses or can I drop down to 5A (yellow) fuses?
3) if I use a 2 relay set-up (one relay for low and one relay for high, like Sucro), do I simply use Fuse Block [1] and [3] and leave [2] and [4] blank?
4) how do I disassembly those headlight wiring harness connectors (that go through the wall near the battery boxes) to replace some of the wires with thicker 12 gauge wires?

Bill,
I can't answer all your questions, but let's see what I can do to help.
Fuse block II, the fuses as stock should all be 8 amps. Someone has obviously stuck in 25 amp fuses without knowing. These are the things that burn up wiring.

Once you have the Sucro relays in you should be able to use 5 amp or less fuses at position 1 and 3.

This because the current through fuses at 1 and 3 now only controls the coils in the relays. Probably less than 1 amp.

Yes, use fuse block 1 and 3 to control the Sucro relays.

Yes no need for 2 and 4, leave 'em blank.

I don't know what size wires are used in a '72.

I would run new headlight connectors with the proper size wire, 12 ga. back to the relays.

Of course you need to fuse the incoming power to the relays.

Are you changing the headlights to something that draws more current?

Sucro should have a spec for the new fuse circuit depending upon the wattage of the new headlights.

Since the relays individually will be supplying power to both lights, one relay for low beam, one relay for high beam, the power into the relay will need to be fused for both lights times the current draw plus 25%.

So if the new lights are 100 watts, you'll need 8.5 amps per light plus 25% or 8.5x2=17+4.5=21.5 amps.
Probably 25 amp fuses.

This is a rather high current example but you get the idea.

Search "sucro relay" for pictures and schematics on how others have done this.

Excellent, Doug, you answered most of my questions. Yes, I am going to H4's with 80/100 bulbs, so it sounds like 25A will be fine.