Congratulations to both of you! We saw a 3650RK awhile back and they're gorgeous!
The duramax/allison will handle any of the Montanas without a problem. You may or may not be over some of the ratings: GVWR total the truck can handle on the truck, GCWR total combined weight of truck and fifthwheel, and GAWR max weight on the axles, particularly the rear axle. If you're not grossly over GVWR and GCWR (quite a few of us are over a little on these) the one to be most concerned about, in my opinion, is GAWR. The 3650 is not the heaviest but is still close to the heaviest of the montanas. The 2004 specs show 11,480 lbs UVW unloaded vehicle weight. Add battery, propane, gear, food, fluids, etc., and that number will grow considerably. The max, GVWR, for the 3650 is 14,000. If you load it for weekends and vacations with maybe a full fresh water tank you will probably find it weighing in the neighborhood of 12,500, maybe a little more. If fulltiming, I'd bet it would see 13,000 to 13,500 or more.
Pinweight is what affects GVWR and GAWR. The brochure for 2004 says 2000 lbs pinweight but brochures are notoriously low. It might be as high as 2400 lbs when the rig is loaded. One owner of the new 3400RL reported his pinweight is close to the brochure weight so I might be wrong on this. Most of us find ours is considerably higher than advertised.
If you're not over GAWR, you might still see a little sag in the rear end of the 3/4 ton truck. Some do, some don't. Some folks have added additional springs or air bags to fix this problem.
There are a number of people towing similar weight with a 3/4 ton truck. Maybe some of them will weigh