First of all, I wouldn't rewire the cab. In my opinion, one of the great things about owning Sunn gear is keeping it stock as possible.....and if your cab is in mint condition, you would be doing a great dis-service by altering it. Why not go out and buy two cheaper cabs - like Carvin, etc., and using those for both of your amps.
I checked the Sunn catalog Thread, and the 610S and 610M, first show up in 1972. As for the power handling, I can't remember exactly, but it seems to me that the 610S cab was rated somewhere around 200 W RMS, and the 610M was a little higher.