If you only putting 5000 miles per year on it, I'd go with a big gas engine rather than the diesel. Diesels are heavier fronted and get stuck much easier.
As far as the chevy gassers getting 5 mpg - that's crazy. My 98 heavy 3/4 ton got 13 empty and 8 - 9 towing heavy. The 454 should get you around 9 - 10 empty and 7 - 8 towing even bigger loads.
I say get a 454 GM or a 460 Ford.
For price comparison, I sold my 98 K2500 for 7500 back in the spring. It just had a 350 engine, but it would handle a load.
That 350 chevy might pull that load but not for long. The diesels are your best bet for heavy haulin no matter how many miles a year you use it. ;-)