[8 Valve Ed]

6mm is getting close but, there is a very easy way to stop the engine moving forward under mild impact or heavy braking; use a Morris Minor gearbox restraint. They had a short piece of steel cable which was attached to the back of the gearbox and was tensioned through the gearbox cross member with a nut and locknut. The reason was to counteract the pull forwards when the clutch pedal was pressed, if the cable snapped as thy did when they had a few miles on the clock, the clutch stopped working because the engine just moved forward instead of pulling the clutch arm.

Else a small bump stop mounted strategically in front of the sump?

Welding the sump is simple enough with TIG but would need totally degreasing in the weld area at least. Oil and TIG welding don't mix. Old aluminium sumps and crankcases can be sods to weld because the older aluminium can be porous and when heated the oil comes out of the pores in the metal and makes it almost impossible to make a decent weld. I would expect a modern aluminium sump to be OK.

If you do decide to weld a corner, it might be wise to bolt the sump to your spare engine block to stop it from distorting with the heat. I would discuss that with your welder, but be aware, they can twist unexpectedly.