from 6.125 to 6.200 rod dif your looking at 075 that in the piston will let move thing ring pack down some but you can use a 043/043 /3mm ring pack or 1/16 1/16 3/16 .i would go with the 1.175 ch with the 6.200 rods. but 6.125 would be 1.250 ch and run 1/16 rings and would let you have lot of room to lower the top ring land down for NOS and you not going that way ? you want to try to stay above more the 1 inch CH if you can