Wow, just found this thread. What a story and a fantastic looking bike to boot! I've gotta ask about those rotors were they for the GL model? I have a extra stock calliper for my K7 and really would like to do the conversion. I'm a newish rider and these 70s brakes are a little scary!
I'd start by adding a stainless line to the factory setup. I've seen the difference such a move makes on other bikes and believe it will be enough to improve the braking on this bike without going overboard.
Now if you still want the double calipers for other reasons, by all means.
drpoyser,
Thanks for the compliment. The front end is from a 1976 GL1000. I swapped to the GL1000 front for two reasons, improved handling and braking. I had problems with my dual disc 750 front end. The GL1000 offers a turn-key solution that simply swaps in place (calipers, rotors, forks, triple trees, etc.). I originally had the stock rotors, cross-drilled by "el cheapo" one of the forum members. I happened to get a great deal on EBC rotors, which are floating rotors and are much lighter, so I upgraded. My master cylinder is not stock either -- but a modern Brembo radial master cylinder. It stops MUCH better than any stock 750 and better than a dual disc 750.
Prez1967 makes a good point. Many members are fine with the stock brakes. At a minimum, I would recommend cross-drilling and resurfacing your used rotor, rebuilding your master cylinder and replacing your old brake lines with new braided stainless steel lines. These upgrades will definitely improve your braking performance.
IF you decide to upgrade to a dual disc set up, I would recommend a GL1000 front end swap OVER a second 750 brake caliper and rotor. Fitment of the second disc and caliper can be challenging to get the spacing right, and requires you to source longer bolts to mount both discs to the hub. The GL1000 front offers some substantial improvements: stronger brakes (bigger pistons) and improved handling (37mm fork tubes instead of 35mm). It is a plug and play solution. While you have the front end off, you can also replace your steering bearings to roller bearings. There are lots of 75-77 GL1000 front ends available (if you buy the entire front end, it will be cheaper than buying the individual components), and it simply swaps into place.
Initially, I used the stock rotors, which I had crossdrilled by member el cheapo. Not leaving well enough alone, I upgraded the rotors to EBC floating rotors. They are lighter and are floating discs (as opposed to the stock discs), and I got a great deal on the rotors.