I have a 73 Duster with a warmed up 360 and a 4 speed. It has a 8.25 rear end with 3.21 gears and I was thinking about changing to 3.55's. What do you guys think and how hard is it to setup the new gears.

Thanks

Installing new gears is no big deal if you have the necessary tools. The 8.25" mopar rear has internal hex adjusters to set backlash & if you don't have the long [~30"] hex wrench, it becomes difficult to finish the job. Ring & pinion gears exchange easily altho the pinion depth spacer is under the pressed on rear bearing cage & it should be removed to determine the original shim thickness [to compare with markings on the new pinion gear]. This requires a sturdy puller & press to seat the new bearing.

If you've done gear changes before the process is very straitforward: remove the old gear set, check the differential case for excess runout, install new ring gear & carrier bearings, install the new pinion, pinion bearings & new collapseable spacer, measure proper bearing preload & insertion depth [after replacing carrier], set backlash using a dial indicator, replace cover, fill w/syn 75W-140 & you're on your way!

The 8.25 will probably hold up fine, if not as soon as it blows find an 8.75, they're not as hard to come by anymore alot of places are narrowing other 8.75's to fit into A-bodies as stock ones werent as common in the body style. In reality if your going to change the gears its not that hard to do but, setting all the tolerances is though, if you dont have the tools already on hand to set these by the time you buy them it would have been just a cost effective to pay someone to do it.