I put dual discs on my 750 K3 and replaced the brake lines with Slingshot SS, black color. They look stock, mostly. My goal was to not change the master cylinder and my un-scientific opinion is that the rigidity gained with the SS lines partially makes up for the extra volume due to the extra caliper. My brake lever pulls in nicely and does not get too close to bottoming out. Feel is fine......... for a vintage disc brake system. I think if I bought a modern bike I'd probably do stoppies until I got used to the modern feel.
Also, the Slingshot kit came with monel hard lines from the calipers back to the rubber lines. For those that don't know, monel is a copper nickel alloy that looks great, will not corrode and is just soft enough to assure proper routing. Good stuff.
My K3 750 project was to make a "mostly stock" looking restored 750 with a few non 1973 changes to suit my taste which mostly comes from the 1970 K0 I had back in the day.