If you want disk brakes and the later big bolt pattern wheels, I'd get a later disk brake V8 car and swap the whole K in there, coupla reasons forthat
If you get a 73?74? car you get the later spool mounts, and if you get a V8 car you won't have to use aftermarket "swap" mounts to convert
You'll get the parts you need for the disk brake conversion, and if you're lucky, come up with at least an 8 1/4 rear axle you can use
One BIG thing is that the 67 idler arm is a one-year only deal and are expensive, so by going to the later K member, you "cure" that problem. All that is a bolt in.
Where are you located? I have a 8 1/4 axle I won't be using, but it's 2.7x gears, you can have it. I'm in Coeur d Alene.
I've seen several Duster/ Demons etc on the Spokane Craigslist, so they're out there. Even if you had to settle for a /6 K the later one would "cure" the idler arm problem and you'd have to either weld/ modify the K for V8 mounts, or else use aftermarket mounts, but that's not the end of the world, either.