[J P Stein]

The 5R I've seen had the wide spigot lands of the 7R and also the external webing around the fly wheel. They don't have the internal webing at the main bearings....nor the oil bypass mod of the 76-77 7R. A 5R is less likely to be distorted than
a 7R. 3Rs (with the narrow spigot lands) are just flat scary when it comes to 90 mm cyls.

That said, I'd use a 7R as mag cases need all the help they can get. Keeping the mag case motors cool is the most important part of the equation, IMO.