What size radiator?

I just bought a 65 D100 that had a slant 6. The previous owner dropped in a 318 for a quick flip. Truck still has the 6 cyl radiator in it. Will this do the job?

It should if its still good/clean, i have a 20 inch radiator(2 row) on my valiant with a pretty well built 340 and it keeps it cool.

I have one & the rad appears to be plenty big enough for a 318. If it is in good shape you're fine.

Depends on what you are thinking of doing with the truck.

One of the things that came with cop spec Mopars was coolers everywhere.

oil, power steering, extra trans cooler, etc. For durability more than anything else. I have found that if you spend a little time with a tape measure and the radiator catalogs you can usually upsize the radiator for minimal costs, sometimes less, than an original spec replacement.

Example, my 87 Diplomat needed a radiator. It already had all the cop spec coolers but a bit of time hitting the books found me a bigger radiator for less money than stock. Seems the 87 Ram truck radiator had even more capacity than the Diplomat HD cooling setup. Bolted in with one small exception, had to run the truck lower hose. Even the shroud bolted right up.

So if you are thinking of a new radiator, keep that in mind.

Cooling requirements are based on the horsepower output of the engine. It has little to do with engine displacement. So a radiator that cools a 225 at 60mph should cool a 318 at 60mph.
That's the real answer.

My '64Dog D100 318poly has a radiator support with a specific size opening for the radiator. If I wanted to use a larger radiator I'd have to cut. Catalog data for those early trucks is lacking. I looked up later trucks and it appears that the later six got a radiator with less width and an extended flange on one side to block off the unused radiator opening. Everything else got a radiator as wide as the opening.

Conclusion: Your truck will run as-is, unless you are pulling heavy loads, are going really fast, or are going up a long, steep incline.
If the radiator you have has a core narrower than the radiator opening, with a flange blocking out the rest, then you might want to consider a radiator that fills out the radiator support opening exactly.

R.