Dave, I don't know about the .450M cases sticking, I really have no need for a .450M, I have three .45-70s and they never stick cases even when loaded to well above factory .450M loads. As far as sticking goes, the only time I have stuck brass is if the chamber is dirty/oily or I load too hot in other calibers. New barrels have extractors, so stuck brass should be a moot point in any caliber unless H&R still has some barrels left with ejectors.
All Handi/Ultra/BC/Target rifle barrels, frames and stocks will interchange, they all use the same components. The only difference tween an Ultra and a Handi is the stock set on current production. Older production had a nicer finish on the barrels, that doesn't seem to be true any more for any of my new Ultras, but I have a few pre '99 Ultras that have real nice barrel finishes on them. Ultras are made by or I should say they are marked Harrington & Richardson, Handis are made by or marked New England Firearms. Some NEF special/limited editions also have laminated stocks, but none are checkered that I've seen. All Ultras are checkered.
Tim