not sure about the difference between z71 and 4x4.
The frames for the 2500/3500 should be the same but also should be bigger than a 1500.
you can probably get stronger axels for the 10 bolt but it will not do alot of good because you are still using a smaller ring and pinion etc..
I don't think lifting the vehicle would change mpg until you start regearing etc..
almost all auto makers use a plastic tank now
if you are going to spend the money a good gm crate motor will probably be the best bet. They will have a better warranty and be just as dependable as any other motor.