The problem with the 700 SX pistons is they have a 20mm wrist pin bore. V- Max 750s have a 16mm wrist pin and the 800s are 18mm. This complicates things. I have seen seen some use bushings from 16mm to 18. It’s much better to have the correct wrist pin bore. Either custom pistons or custom rods.
The whole crank out of phase issue is way overplayed. What kind of belt did you replace the blown one with? The majority of issues you describe are clutching. There is not enough information here to diagnose accurately.
Applying logic, if the sled ran fine before blowing the belt and the belt...
Nowadays, you have to run the 8BU-17641-01-00 on all Vmax4’s regardless of secondary. That’s all you are likely to find. They will work perfectly. The secret is to clutch accordingly to insure the clutches grip the belt hard enough; usually add weight. These are very “hard” belts, hence they are...