I am currently running a 3.42 drum brake diff from an 88 Camaro..2.8L V6 in my 54. It is 62 " from wheel mounting surface to wheel mounting surface....same as the AD truck. It required cleaning off the Camaro mounting hardware and welding on the perches. The rim choice with this diff and front disc conversion was 5 on 4-3/4 with 4" backspacing. Using the 5 lug on 4-3/4 conversion, eliminates the expensive roller bearing upgrade and uses over-the-counter GM bearings, disc/hub assembles and calipers. However, it makes it tough to find salvage rims that meet the specs. I used aftermarket 15X7's on the front and 15X8's on the rear with 235 and 255/70's respectively. This rim clears the discs, the stock, as well as upgraded tie rod ends on the front, and centers the tire in the wheel well in the rear. Given it to do over, I would go 235/70/15 all the way around with 15X7's so I could rotate the tires.
These diffs are found in 3.08, 3.42 and occasionally a 3.73 and the 89 IROC-z is a posi/disc setup with the 3.08 being the most common. It is an Australian built Borg Warner that is used behind a 305 V8....the replacement brake calipers and seals are expensive, but it is another option. Hope this helps.
Dave
