Ah, good points. I figured there was some reason the A bodies had a different size than others, but wasn't sure. And...did the slant 6 have a different fan than the V8? Questions, questions. :)
the slant fans are likely different due to space limitations. somebody here would certainly know the granular details.
i've long run clutch fans off of later A's as well as F,M & J bodies without nary a problem.
the only problem i could see with a 20" unit is clearance for either the hose/trans lines or no room to get a shroud in there. but that may be a non-issue with your set up seeing as you're later bodystyle with different radiator-- my experience lies mainly with the earlier stuff.