I don't think the insulation foam would last 100 miles without wearing through.

Depending on the thickness of the case grinding a bit could work.

I think nudging the tranny mount over a bit would be best.

You only need 1/8", it not like that case is going to move a bunch.

Also, are those stock 4wd torsion bars? You can switch to 2wd bars and gain a couple of mm of clearance. Not sure how they would handle the extra weight of the 4wd though.