Well, his description does say "curmudgeon".
Anyway -- delivery could have been milder, but he IS right, to an extent. I believe the question could have been better... more like will the 4.0L EASILY fit.
Enough of that -- the answer is YES. The 64+ Ramblers will take just about anything. As several have pointed out, big blocks are really tight, but they were designed from the factory with the then under development AMC GEN-2 "small block" in mind. The bigger cars were designed to hold the GEN-1 AMC/Rambler V-8, which is about the same size and weight as a Ford Y-block/Chevy 396, so a big block will fit them just fine. The 64-65 American engine bay is a couple inches shorter than the later models though. There is a mod to the radiator brackets and upper support filler panel (remove the filler) that will gain 1.5-2.0" though. That's all you need for the 4.0L. Might have to use an electric fan or do away with the clutch and use an aftermarket flex fan (on a Wrangler or Grand Cherokee water pump -- fan won't fit an XJ Cherokee pump and the bearings aren't made for it).
To use a 199/232/258 a CJ-5 (NOT CJ-7!) water pump and pulley are required. They are a bit shorter than the standard AMC six water pump and pulley. That pump was originally designed to fit the 232 in the 65 American, worked for the short CJ-5 too.
AMC "big" cars are the Classic/Rebel/Matador and Ambassador. Everything else is considered "small" and use similar parts and have similar sized engine bays.
The little 58-63 Americans and 50-55 Nash Ramblers (the same car under the skin!) are a totally different story. Those cars were literally built around a short and narrow L-head six, so the bay is tight! Already spoke about that though...