You can get a radiator cap w/ built-in temperature gage on ebay cheap. If it isn't boiling over, you have no problem, just a concern. You won't hurt a slant by boiling over (did that many times). Cast iron heads are very forgiving.
If and when you do need a new radiator:
1. Your 65 should have a 22"W opening. They might have put a narrower radiator with a filler plate in it, like they did in later years. Even if you have a 19" opening like earlier Darts, I would use a 22"W radiator. 18"H is standard for all years. You measure just the core dimensions, not the tanks and brackets. There were some 19" H Mopar radiators in B or E bodies, but slightly too tall for our cars.
2. My 65 Dart has a radiator for a ~1971 Dart from Autozone. You can see it in my avatar. That was the first year they listed. It has a 1-1/2" lower tube on the passenger side, which is correct for a slant. The tube isn't angled, like most slant rads, but should work. I have a 273 in that car, but an alum water pump so the tube matches, though I use a rubber bushing to get to the 1.75" D of the water pump.
3. My 64 Valiant slant has a radiator of unknown source, but it works. Certainly not original since a rectangular upper tank that is narrower than the later Mopar "square tops". The lower hose that works for me is Goodyear 60664 or Dayton 70776. It bends behind the alternator belt just right. Indeed, that same hose also works as the upper hose in my small block and big block cars.
