Hmm... mounting my Marathons was a snap. I guess some rims are different than others. Are you positive that you got the right size?
Anyway, rubber actually contracts with heat so maybe finding a cooler place or even a pre-mounting cool off in the fridge would give you that last few centimeters. I think some combination of lubrication, using more tire levers and locking them to the spokes, and some brute force should get you there. Mount them as far as you can and lock it down with the levers and let it site a while to stretch and save up your strength for a good all-out effort.