I've seen them run with .250 but you really want to get it down to .015 - .025 for it to be "safe'. Many times this can be achieved simply by redrilling the main bearings which are just basically "aluminum bushings". The bushings are located axially by a dowel that also serves as the oil passage to the bearings. The trick is to center the crank so that the rod journals are evenly balanced between the two piston bores, then mark and drill both bushings in a new location (turn the bushing to present a fresh surface to drill). No parts needed other than fresh gaskets. If the radial clearance exceeds maybe 010?, you can also just fit new bearings/bushings. To me, taking care of that is preferable to dropping a rod and damaging other parts, including the magneto shaft which can shear as a result of excessive crank clearance

That's a common symptom - the dowel being flogged out from the pony crank sliding back and forwards as the tractor clunks across the field. Obviously, "standard" wear will occur as well but the end float is usually from the dowel hole (and sometimes the dowel itself) flogging out. Good practice to replace teh dowels - they cost nothing last time I looked. On my 5U, I elected to remove any reliance on them and instead milled each end of the crank and the adjoining faces on the pony block/front cover to accept a thrust washer. Now any clunking to and fro by the crank bears on the thrust washers and not on the dowels

Yes, they both wear more on the inboard side because it's that side that takes the impact when the crank slides. Some kind of a bracket on the clutch cover, done well, would probably do a good job of holding the flywheel/crank so it couldn't clunk back and forth. Do post pictures if you make something.