With money being a concern, there are a few caveats to what you want to accomplish. There's a lot to it that isn't always evident.
If you want to keep your existing 5 on 4" bolt pattern, the factory setup seems the obvious choice. However, calipers and rotors aren't always available--they seem to come and go--and the lower ball joints, at least quality ones like Moog, are no longer available. Yes, you can ream the bolt holes on the drum-brake lower ball joints, but A) that's an added expense and B) the drum ball joints are going extinct too. Moog has discontinued those as well. There are some overseas-sourced drum ball joints available, but they're still pricy and the quality is sketchy. The Moog (and TRW, back in the day) parts were USA-made; I hoarded every disc-brake Moog lower I could find since I'm now my own warranty provider.
Swapping to the 5 on 4.5" bolt pattern is a double-edged sword. All the service parts are much easier to find and less expensive, which is a definite bonus, even though you need new or good used upper control arms. The calipers are also less fussy. The downside? You'll need new wheels and new rear axle shafts, or a different rear axle with a 4.5" bolt pattern, unless the inability to correctly rotate the tires (easily) and carrying two spares is appealing to you. Redrilling your existing axle shafts and drums is not recommended. If you've got a 7.25" axle and are unable to find large-pattern axles (or a complete assembly) you'll need an 8.25" axle, which requires a shorter driveshaft and different shock plates. If a wheel swap was already on the menu, the world is your oyster with the 5 on 4.5" bolt pattern. Since you didn't mention that, I'll consider the rear-axle complications a downside.
Although the initial buy-in seems hefty, realistically the Dr. Diff setup linked above is probably the most financially sound one. Why?
Both 4-piston small-bolt and big-bolt conversion parts have an initial buy-in from a board member, swap meet, etc. which is probably $250 or more. Unless the seller has fully reconditioned them, you're getting all used parts, which means you'll need to pop for calipers, pads, wheel bearings & seals, hoses, ball joints (all 4 on a big-bolt swap), and on a big-bolt swap, probably upper control-arm bushings. The big-bolt swap will also require an alignment. Even if the rotors are OK they'll almost certainly require turning, so there's some rotor expense either way. While you may be able to cheat with your existing master cylinder, you'll definitely need a either a disc/drum or adjustable (rear only) prop valve. All this stuff adds up quickly, and your time has value too.
The Dr. Diff kit is a 100% new, 1-stop solution (OK, the calipers are remanufactured). You have your choice of bolt pattern, so you can keep your existing wheels and never even look at the rear axle. If indeed everything but the spindles ships from RockAuto as mentioned above, then all the wear parts are easily acquired. If you include a new master cylinder and prop valve, you're under $900 (not including shipping) from Dr. Diff using rear-mount calipers with the late lower ball joints and adapter sleeves for the uppers. Everything you need lands on your doorstep--no chasing around, no asking questions on which [insert part here] you need or finding out you're still missing parts mid-job. To me, given a similar situation, this is a no-brainer.
One of my best friends wants to do a disc swap on his daily-driven '73 Dart, also native 5 on 4", which he wants to keep. After looking at the expense and semi-dire parts availability of the factory K-H 4-piston swap--mind you, he works in a parts store and gets parts at cost--he feels the Dr. Diff swap is still the better option, especially down the road when he needs service parts he can easily acquire locally.
My future-daily '68 Valiant, on the other hand, is going to the 4.5" bolt pattern based on the availability of new parts and sheer good fortune. I lucked into a number of absolutely killer deals, including a dirt-cheap 3.23:1 Sure Grip A-body 8.25" rear axle, but it also took more than three years to gather everything. If I hadn't had the time, I'd probably have called Dr. Diff myself.
