Slant six noob

Welcome. I have you bracketed with a 65 Dart GT 273 and a 64 Valiant 225. Ditto on fix it yourself. A shop will bleed you quick. Don't blame them. Restoring a car takes much effort, and would only return $2/hr for your labor. Most of us do it as a hobby (maybe a few $2 wh--es). I watch Wheeler Dealers a lot and figure they make next to nothing for their labor (3 people, untold hours off-screen). The plus side is the knowledge you gain lets you fix any car. I save a lot doing little jobs on my new cars, that would cost $$$ at a shop, and even a few biggies in rebuilding the transmission in both my 2 newer Mopars.

Post more about your brake conversion. Sounds like front disks. Some kits are >$1K.

Re overheating, all good advice. Since it overheats on the highway in 30 min, you have serious blockage. As a kid (maybe 1967), I recall our ~63 Valiant station wagon slant boiled over on the interstate about 30 min from home. A road worker came over, opened the cap, looked in while the engine ran, and said "no water flow". He removed the thermostat and found it stuck shut. Told my dad he didn't need one in FL and put it back without. We continued our trip with no problem, and knowing my dad he never put a thermostat in (I would). Hope your problem is as simple.

If you decide to change the radiator, run a strong flush through the engine first, with the old radiator still in. My 65 Dart had a torn radiator, so I hooked up hoses to bypass it and ran the flush direct thru the engine. That really concentrates it and heats up fast. I put a stainless screen in the top hose to catch debris (very little).

I got a new radiator at Autozone ($200), for a 72 Dart (recall), the oldest listing in their book. I now think for a slant since the lower hose is on the passenger side with 1.5" tube, which should match your slant. For my small block, I had to change to the later alum water pump and use a rubber bushing to match its 1.75" inlet. I had to drill a few holes in the radiator flange to fit the Dart, but otherwise fit fine, other than it sticks a bit farther out from the car's frame. I found a plastic shroud that bolted right on. Many people have installed an aluminum radiator, which fixed their over-heating. Shop around. I got a new alum radiator for my C-body on ebay for $80. For the lower hose, I recall Dayco 70776 but check my posts for the real PN. If it doesn't have one, make a spring from stainless wire and put in the lower hose so it won't collapse under suction (could be your problem). You might need a different upper hose for the 72 Dart radiator, but that is easy to fit.

My 64 Valiant has an unknown radiator, but it works. Could be for a 70's Dart, or maybe even non-Mopar. If you ask, I can post photos.