Dogsled

So our club is getting our second set of custom kits from Hincapie. The first go around it was unanimous (except for me) that it was bibs not shorts. I never used bibs before but they make the kit so I ordered size large. They physically fit ok but after 30 miles things are not good in the chamois dept. In a similar situation with regular shorts, I would just pull down on the leg sections for a readjustment and all is good. With the bibs, the shoulder straps prevent you from doing that.

My regular shorts are Castelli med and fit good. So for this order I am not sure whether to go up a size or go down a size. What if you had 2 riders both the same waist size but one was taller than the other. Would the taller guy have a worse fit due to the shoulder straps? I tried both XL and Med and standing there they both felt ok. Where should the focus of fit be? What is your experience?