kits available for disc brakes

If you really want to keep your 4 inch bolt pattern, and go to disc brakes, there is another way to do it if you want to go that route.
Locate a rebuildable 65-72 Kelsey Hayes disc brake system, if one could be found anywhere.
They are small, 4 inch bolt pattern.
I used to sell quite a few rebuilt set up's through the for sale pages on this forum.
But then you would have to change your upper control arms over to 62-72 UCA's and use disc brake lower ball joints for the 65-72 cars.
Would be expensive, but it can be done.
I have seen one car done like that, because the guy already had the majority of the parts to do it.
Upper control arms are a dime a dozen.
Every car of those years had to have them, so no big deal, it's the lower ball joints that get expensive.