Master cylinder

There are all sorts of things you CAN do, but the way you SHOULD run with disk brakes is with a disk brake master cylinder and with a proportioning valve. The PV was built-in to the junction block/safety switch on factory disk brake cars starting in 70, so you could just buy a replacement unit -- pure bolt on. This unit is non-adjustable, but it is likely to work OK so long as your weight distribution is not radically different than stock. Or, it was a separate part, inline in the rear brake line, on factory disk brake cars 67-69, but this unit is is not repopped AFAIK. It would be the hard way to go, anyway, since it requires splicing a brake line, and although it was theoretically adjustable, it didn't have much of a range (it was usually all the way in or out). Or, you could buy an aftermarket proportioning valve, but again, it would require messing with your brake lines. However, it would actually give you a wide range of adjustment if that is an issue.

BTW, the proportioning valve doesn't adjust the AMOUNT of braking force on the rear, it adjusts HOW QUICKLY it comes in, to avoid lockup. If you need to adjust the amount of rear braking, that requires changing the rear wheel cylinder diameter.