Radiator fit

I have two radiators that came with one of my junk yard vans. The van Im working on is a 67 A100 that originally had a 225 and a727 in it. The PO had swapped out the 225 for a 318. Thats what Im putting back in. Ok now the radiator problem, one rad is a bit thicker and it has an internal cooler for the auto trans. part number 2952843. The 2nd rad part number is 2508938 it is abit thinner and has an external cooler for the auto trans. My gut tells me the thicker rad is the one that should be used. However the mounting flanges on one rad are in the front of the rad and the other one the flanges are in the back of the rad. Unfortunately I did not get to see which one was being used as the van engine comprtment was disassembled when I got it.
Are external trans coolers any more or less effective than the internal ones? Any suggestions?

Hi- use the one that fits- most likely the front mount and =YES!! the add on trans coolers work better as they are colled by air flow - not water thats hot - also run the add on trany cooler after the radiator cooler to gain more temp drop for the trans fluid.

Transmission fluid has a 'happy' operating temperature, just like engine oil.
Factory radiators with the trans cooler built in while cheaper than a separate cooler are also designed to keep the ATF at this happy point. If you plan on heavy loading (towing, high speed, drag racing) where the ATF temps can be higher than normal, an additional external cooler is a good idea. Trans guys I have talked to have suggested running the fluid through the external cooler first, then thru the radiator, again to keep the fluid at that happy temp (not too cold).
I seem to remember the V8s having a different top hose placement (to the right) than the /6 (middle?). I don't know the part numbers, hopefully someone will post on those. I looked through a bunch of the ad photos I've saved, and all /6 seem to be just to the right of the filler cap, V8s are to the left, right and a few to the same point as the /6 (swaps?).
I will say that the /6 is longer than a V8, so it stands to reason that the /6 radiator would mount further forward.
Whichever radiator you use, have it cleaned and pressure checked at a minimum. They are not fun to R&R, and you want the van to cool properly.