Welding on the balancer hub will probably ruin the rubber inserts in the damper. Use a speedy sleeve instead- - - -the same one that fits the small block V8 also works on a stovebolt balancer.
I just measured a 216 balancer with the hand crank lugs and a 5/8" belt groove and it measures 5 3/4" at the OD of the pulley sheave. That translates to an 18" circumference at the outside edge of the pulley sheave. Your 6 3/4" sheave will have a circumference of 21". That's a 16% increase in belt speed, which probably won't make a big difference in the speed of the water pump, which is already turning faster than the crankshaft even with the smaller pulley. A 1/2" belt, which is probably what your water pump needs to run, won't ride at the top of the 5/8" belt groove anyway, which will slow down the belt speed somewhat.
Jerry
