Suspension component selection (and "tuning") is a function of vehicle mass and distributions. Indeed, it is not the converse.
The polar moment is a complex calculation, most likley beyond the scope of this forum. Using the classic textbook case of a rotating beam as a guide to an understanding of this concept is faulty, as it ignores the complex dynamics of a vehicle subject to a twisting (torque) at the "four" endpoints of this "single" beam.
As simplistic examples of the complexity of the calculation, consider the cases of a rotating sphere and a tumbling cube. For the sphere, the polar moment is obvious. For the cube, the polar moment changes depending on the instant in time.
*** At this time, nsxman calls on all calculus wizards to standup and be counted. ***
In Honda's engineering wisdom, they placed the spare tire close to the center of the vehicle. We can guess that its removal makes only a slight difference in the moment calculation, as the mass on an empirical basis, appears to be on a centerline with, and near to, the fore-aft locus of weight distribution.