Unfortunately someone in the tractor's past fixed a failed front pump drive by welding directly to the splined end of the crank, preventing removal of the nut without cutting the weld off.
From your other posts, if the 4wd tractor's engine is being taken out and replaced by the 2wd's engine, I'm wondering why this front cover needs to come off?
The short driveshaft for the loader's front hydraulic pump is a long-discontinued item for these tractors. People usually improvise with some kind of a set-screwed couplings, and avoid welding on the crank end.
Is this engine in running condition now? If so I'd use it as a lathe and machine the weld area down to original spline diameter so the nut will come by it, then cut the rest of it off. If a pump drive needs to be reinstated, it might be possible to make a flanged driver that centers on what's left of the crank end, and gets its drive from bolts in the pulley.