An SSM IV is a shorty 3. An SSM V is shorty 3A.

I don't think that is correct. Larger gear sets . You can't handle more hp just because the drive is shorter. The higher the # the more hp you candle, that's the simple way to explain it .

The vertical shafts could take more torque being shorter

The #3 and #3A are basically the "same number", but way different in internal construction. The #3A has the same straight cut, near net forged upper gear set as the #5 and the same propshaft (1.75" dia.) and lower gears as the #5. The #3 and the "shorter" surfacing #4, share, basically the same internal components. In my opinion, the "weakest" of the four drives is a #3 followed by the #4, then the #3A and #5 are the same strength, but the #5 is the "surfacing" version of that set.