Earlier front calipers use a section of hard line...banjo bolt won't work, thread is the wrong size and sealing is done by a reverse flare to seal against flared tubing of hard line. Slingshot (and probably others)has the needed adapter, but it will probably be from the flare to -4 AN/JIS. So then you would need a brake line with banjo on one end and female -4 AN/JIS on the other.
The GL mc is not necessarily the easiest solution. When I needed a mc, all cb750f masters were north of $100 in crusty, unrebuilt condition. I paid $60 for the GL, ready to go. I needed a brake line anyway and Slingshot knew exactly what I needed and set me up. When I was looking on ebay, I found lots of cheap mc's from '80's Honda's, such as DOHC cb's, that had that diagonal bolt pattern...but I really have no idea if they would be easy to make work. Do you have access to a motorcycle salvage yard?
I got into this situation for the same reason as you. My original mc was just a lump of solid rust, hardened brake fluid, and corroded aluminum. I have not had good success rebuilding mc's anyway. After using the GL mc, and on a whim, I tossed my original mc into a coffee can full of vinegar, and forgot about it. A couple months later I stumbled upon it and fished it out. It now moves freely and is as clean as new born baby.
