I don't think there was any difference in the Rudge type splined hubs used by Morgan. The stub axle sizes changed, but that doesn't affect the wheel hubs.
I reckon a set of modern 72-spoke wheels will be a direct replacement for a set of 60-spoke jobs - always assuming that the rim widths are similar.
I believe 1961 +4s would have had 5" rims, so the only ting that might cause an issue would be if the '91 car was a 'wideline' with 5.5" rims - and the balance of probabilities suggests probably not.
1986 4/4 VVTi Sport, 2002 LR Defender, 1957 R4 CV, 2005 Ferrari Vipar