The 8.1L was discontinued in '07; too bad, it was a great engine. If you want to stick with a gas engine Dodge is introducing the 6.4L hemi in the trucks this year but that means buying new and being a "guinea pig". The 5.7 hemi is a fairly proven power plant but it's no big block.
Does GM offer the 6.2 in a 2500 or 3500? I thought they were only in the 1500 trucks. (Which really wouldn't make much sense.)
The Ford V10 would be my choice if I had to have a Ford. Otherwise, their 6.2L is what's available now.
Unless you don't pull much at all and/or let the truck sit a lot I would think you'd be happier with a diesel for that kind of weight. A GM Duramax pre 2011 would not have the DEF requirement and should be plentiful and reliable. The Dodge 6.7 Cummins has been a good engine for quite a while now. If you're buying a Ford diesel don't dare get a pre 2011.