Higher gear for differential

[eyebuzz]

Hello. Looking to improve MPG a bit - currently getting 7city/12hwy (would be happy with 10/16+). And beyond MPG it would be nicer to "cruise" a bit more on the highway - currently "force" is a better word to describe the ride on an expressway - even at 55. '72 c20 Suburban with 350/350 - so it probably has a 4.11 - 4.56 in the differential. I don't plan to tow (and probably never tow anything over 1,000lbs - but will, on occasion, haul 1000+ lbs in the truck over hills and such)...on to the questions:

1) To me, it seems easier to change the gearing to something higher within the differential than to look for/install a whole new rear end - but can this been done? (from everything I see here, yes - but a mechanic told me it can't be done...so I'm here asking the question directly)

Assuming, yes you can change the gears in the dif:

2) Can I go as high as a 3.07 with no issues (being a c20 was my transmission set up differently than a c10 - that kind of stuff)?

3) I recognize this needs to be a professional job - any idea of cost ranges (at what point will my picture and the word "gouge" be synonymous)?

Thanks,
TT