'72 Chev long horn,3/4 ton,350/auto NEED REAR END INFO

[oldasi]

Have owned this for about 10 yrs and just getting it on the road. Came with no rear bumper and appeared to have had a heavy duty hitch welded to the frame that was removed, had a brake switch mounted under the steering column.
Seems engine revs quite high at cruising speed of 60 mph, just sucks up gas like crazy.
How can I determine what rear end ratio is in it ?
Whats available in ratios. ?
How big of a job is it to change to a different ratio, ? I had someone do the brakes a little while ago and I believe they had to pull the rear axles out, rears are drums, front disc
No doubt will have further questions