used gears are great. just take the rear end to a competent differential place and get the depth and pattern set. (it is money well spent even with new gears.) setting up gears is a job i CAN do, but it sucks without the proper equipment.
i have a used set of 3.55 gears in my 65 B-cuda that came out of a 1999 Dakota, so its basically the same exact set up as yours. if you can get a sure grip, do it now, set it up once, and be done with it! i am putting a sure grip in mine next weekend!
sb