My '85 Bridgestone 400 came OEM with the ARX-GT rear deraileur, a 14x32 6-speed freewheel and an SR crank with 52/42 chainrings and tapped for a granny ring which I added. It worked fine so I expect a 28T cog will be no problem at all.

Generally, the long cage versions of a rear derailleur will clear the same maximum rear cog but just won't "wrap" as much chain. So the ARX-GT handled the wide range freewheel and the triple crank but the non-GT would have problems with that many "total teeth"even if it cleared the same large cog.