
Hey I would get the gears installed by a guy that knows what he is doing...
was the 600 including all new barrings? Fluid? Seals? and all the other goodies that u should change when the rear-is out...
I just had mine done.. it was around 350 for labour... and I had all seals barrings, cups, nuts, replaced, including a new possi diff,, for my 31 spline axels.
I went with 4:10's but I'm running a Tremec T-5 with the different 5th gear.. which brings my RPM's down when I'm on the highway.. at 120km I'm only @ 2200 to 2300 RPM....
With my 331 stroker.. AFR 185's.... cam.... and all other goodies I can still get good highway milliage.... I figure around 20 miles to the gallon easy..... if not more
Then again I have my car tuned in with a TwEECer RT also... so that can help with the gas milliage...
If you change gears go with the 3:73's insted of the 3:55's there will not be much of a difference in RPM & gas milliage.... and u will just want the 3:73's later on down the road if you do not go with them 1st...
I've been through all sets of gears.. 2:73's 3:55's 3:73's.. and now 4:10's and would never go back unless I needed to becasue of a turbo setup or something..
Good luck