I agree with everyone here on things to address with your 125. That K-321 is a bigger and heavier engine, and therefore, it is more critical that everything is in perfect alignment to work properly.
A stainless steal driveshaft isn't necessary. If you are confident that the driveshaft you have is of the correct length, then look at the couplers and holes in the driveshaft to be sure they are perfectly round. Wallowed out holes on any of these parts can cause movement and a slam on your spirol pin. Those pieces need to be replaced. If you need a driveshaft, a machine shop can make one using 4140 shaft steal. That steal is a bit stronger than stainless, and a bit cheaper, too. As previously mentioned, check your couplers for wallowing, too.
Another option is to find a 129 or 149 rolling chassis, and transplant the engine. There will be no modifications necessary, unless you want to change the driveshaft to the Quietline style, using flexjoints. You'll love the wideframe design.
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