PSIG, I'm guessing my 66 stock type is a 1G or 2G so the newer 94+ G3-G6 would be better for sure. You said the two wire one is for a dash light if used.....where do you hook up that second wire? I'm guessing some wire off the old regulator? If you have a volt meter gage wouldn't that tell you your alternator has gone bad just like a dash light would?
I think I'll check into a 94+ G3 model off a 95 Mustang and one more question....what do you do with the old regulator wire harness hook up?
Stock '66 is 1G. A decent design, but fairly weak from that time period of few power accessories. The 1980s saw 2G and the GM swap was popular because it was better, but had a tendency to catch stuff on fire. While more powerful, both the 2G and the GM series required higher RPMs to make rated power. This is why under-drive pulley kits were popular with racers and deep-geared street cars, as the factory sized the pulleys to spin them fast to make the necessary power at lower speeds, and wasted power at higher RPMs.
Ford went overboard to fix issues and prove they could make a superior alternator with the 3G - and they did. Spinning it slower, the 3G could make more power at idle than the 2G or GMs at highway speeds. They had a winner, but like all things new, it was very expensive at the time (much like 6G is now - even better but still expensive). So now you know more than you wanted to about the history of Ford's designs.
The 3G comes out-of-the-box as a standard 3-wire hookup. The wires are the BATT+ cable, a voltage sensing wire, and the red/grn for your dash light. The V-sense wire should go to the starter relay to monitor system voltage most accurately, but many have found simply attaching it to the BATT+ stud works fine. Now it's a 2-wire. If you don't want a dash light, you just don't connect the red/grn wire to your old red/grn wire. That makes a 1-wire hookup. Stock.
I strongly suggest you take the time to do the 2-wire hookup, as having a warning that there is an issue can prevent you from being stranded somewhere with no warning and a dead battery, or an overheated car because you threw the belt and didn't know it until you noticed steam clouds. It's only one wire. I'd take the 30 seconds and hook it up where the old wire ends at the regulator connector, even if you have a volt meter or ammeter.
You can trash the old regulator and much of the old wiring. Or, I left everything there on my last car so it looked 'stock' at a glance - it just didn't function and was there for show. Super-easy hookup too. I never had a single person catch-on that it was a conversion. Do a search on this forum for articles and several threads about conversions so you're comfy with the process. You can also post a copy of your schematic, and we can mark it up to show all the stuff you can rip out if you want to do it that way. Have fun!
David
More power and more efficient than even higher rated GM types at normal driving speeds, bolts right on without adapter spacers or bracket mods, and available at any parts store or wrecking yard. Oh - and it's FORD. 
