I THINK, based on the info here, that the 10.75 neched down or the 9.3 necked up would be best...

leaving the 10.75 a 10.75 has got to be hell on bullet selection...

if you're looking for something soft on teh shoulder, either the 64 or 68 case would be reasonable... IIRC the 68 is ~10% behind the 64 in velocity in the loadings I've seen... (I'm probably way off, I've never paid much attention to the european cases).

if the 68 case would allow you to get an extra round in the magazine, go with that.