Will not work. The crank throws will hit the dangling rods, 600 grit will take a week to do one crank pin. There will be no oil to lube the main bearings and grit will be slung all throughout the engine lower. The crank pins will not be uniformly cylindrical.
Rent an engine hoist, yank the motor and do it right. Renting the hoist for a day will be cheaper than the cost of that sanding roll All this pfutzing around trying to re-machine a crank while it is in the car will take loads more of time and won’t work anyway.