HR gave you the ratios.
I would go to a site and figure out what rear ratio you want, you might as well do this sort of thing once and you're done
http://www.smokemup.com/auto_math/mph_range2.php can do this for you. you input the tranny and 'pick' a ratio. I live in the foothills, so mountains are an issue, I have a 3.40 w/ a T5 speed. Your choices are wide if you need to change the gearing on 10 and 12 bolt chevy rears, mine came from a 77 chevy suburban 4x4. For towing the lower (4:10), or speeding on freeways (2:90) to 1 is a benchmark. I had to cut all hangers off the axle and weld on a conversion kit from chevy duty, which moves the axle 1.5''. We actually moved the axle 2.5'' so my 16 gal under bed tank would fit. You can use the ebrake cables that come w/ the 10/12 bolt w/ little mod.
I also believe the rear swap will be BETTER than rebuilding you old rear end and re-gearing.
we had to cut the old hangers off completely so the new kit could be welded on to fit our 51. the kit allows you to move the axle back to 'center' it in the tire well by 1 1/2'', WE DID 2 1/2'' using another 1/4'' plate and redrilled the index hole. It looks great.
