While I think Stens belts are fine, I would never use them in this drive system. You will read over an over again about using the OEM MTD or Toro belts in these drive systems. MTD builds your rider BTW. Also know that the cut of the belt is different than a typical V belt. Now, MTD also always said to replace the belts only in sets. My experience is that I've found it OK to replace the rear only if the front seemed good. Replacing the front only isn't a good idea unless the rear was just replaced. So if the bearings in the VS pulley and idlers are good and routing is good get a new set of belts and I suppose you could try a new rear and see what that does for you. If not better- then install the OEM front. There may also be a pin in the center on a 600 series that the front belt has to be pulled over- so be mindful of that on install. 112-0301 I believe is the same as MTD 954-0467 and 112-0305 is the same as MTD 954-0468.

you were right put mtd belts on and everything is fine

cool. Thanks for the follow up.