by "guy in Michigan", i assume you mean www.AluminumV8.com
i don't know what you mean by saying that the Rover is "smaller". 3.5 Liter is the metric displacement equivalent of 215ci the same as 7.5L is the metric equivalent of 455ci. being that the Rover was produced to metric dimensions, obviously there are going to be minor variances in part dimensions and i wouldn't try swap something like pistons or bearings without having a machinist check / rework everything but Buick parts are used with Rovers all the time. one particular favorite is using a 300ci crankshaft to stroke the Rover blocks.
Rovers, as a practical matter, are the same size externally as 215ci and have the same bell housing pattern. and this includes the 4.6L.
i can't speak to the variances in crank diameter but i'm sure people on BritishV8 would know all about this. www.TAPerformance.com may also be able to answer this question as they have a set of high performance heads available for the Rover / 215.