There were several block castings (number is on the lower right side of the blocks) for the F134 starting in 1950 and running through 1971. The number that tells you what model chassis the engine was originally built for and installed in is the serial number on the small flat above the water pump. The prefix ID's the chassis IE "T" = truck, "U" = Utility/wagon, "J" = Cj, MD = M38A1. 4L was a very common prefix from the late 50's on and didn't help ID the original chassis anymore. An extra "R" in the prefix meant the engine was a brand new replacement engine from the factory.
You said the CJ5 engine came out of your friends M38A1. You should compare the welded steel bushing length on the front engine mount plate for the generator mounting to determine if the plate is CJ5 or M38A1.
You should sort out the front engine plate issue right away. If you do need to switch the plates it requires pulling the timing cover and the timing gears. Make sure you use the correct puller on the fiber cam timing gear or be prepared to buy a new gear. You will need several gaskets up front and you should have the book on the bench when you start.
The radiator and tank shops will go as far as you tell them too. If you are confident they are both corrosion free and only need a cleaning then of course go for it yourself. It won't hurt to take the radiator to a shop and have them test it under pressure and flow check it.
Cuz (AKA Wes K)
45 MB, 51 M38, 54 M37, 6? M101A1, 60 CJ5, 76 DJ5D & 47 T3-C