We just changed our king mattress out for a queen mattress. The new queen is a memory foam type mattress 60"X80". This gives about 6" more room on each side of the bed. We wanted more room on the closet side of the bed. This sort of thing is more of a personal choice.
Most of the larger mega furniture stores have sales near all the holidays if you are looking for a new mattress.
We went with a home style mattress, a Sealey Posturepedic pillowtop. In our 3400 the mattress is a standard queen size, including length. In our prior Montana, we had the short queen. We also replaced that one and the mattress store had one built to our length requirements at no additional cost. Any mattress you find comfortable is the one I'd recommend you buy.
If you have the King with the rounded corner, then I can't help.