As far as I know, the Spicer units are not side specific. They were comprised of standard Spicer parts - supposedly now discontinued.
They DO need to be in phase, though. Make sure the u-joints are in line with each other, or things will be not be smoothly rotating.
I have never heard of shafts this short being balanced.
And do realize that the orientation of the shaft does not mechanically matter, but you can gain exhaust clearance by putting the smaller diameter shaft above the exhaust piping.
Larry
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