I take it that it is the rear U-joint since most 350 trans have a slip yoke. It also must have straps instead of U-bolts holding the U-joint to the pinion flange. Just cut the head off the bolt or cut the strap and then if vise grips won't remove the bolt pull the pinion flange and use a drill press or mill to cut out the stub of the bolt. Square off the end of the stub and drill EXACTLY through the center with a 17/64 drill. This will leave such a thin remainder that GOOD 5/16-24 tap will clean out the old bolt to the original threads. Don't worry about the crush sleeve just torque to a few #s over recommended spec and smear with red loctite.

Evan