My 6 bolt Budd converted rims (originally 10 bolt budd) - 10 stud 3/4" diameter use a 20" o-ring with a 1/4" cord diam, the 20 stud 5/8" diameter rims on my M-543a2 use a 20" o-ring with a 3/8" cord...
Finally I have one spare rim that uses a triangular cross section ring which is molded rubber - it is 20" at the mean diameter but a low durometer 1/2" cord o-ring will work as well... (it is a 10 stud 5/8" diameter rim)
All the wheels just seal at the curved joint between the lock ring and the shell - my 5 ton has the bolt pattern in the lock ring, the converted rims have the pattern in the shell.
Either way you do it make sure the rim is polished and clean on those surfaces and use some o-ring lube for the ring - I run my rings over cured paint as it helps seal and fill any small imperfections in the metal.
HTH
Matt