I'm running a 3.42 88 Camaro 5 lug diff in my 54, with a 235/.72 T5, which is 62" from wheel mounting surface to wheel mounting surface...the same as the stock 54-55.1st. If you're planning on building a driver, you'll want to consider front discs. The 5 lug 89 IROC-Z (62") has a 3.08 w/posi and discs that might be just the ticket. This diff can handle up to 300HP comfortably. It was also available in 3.42 but the most common was the 3.08. Dependent on what transmission you're planning on running, these are a couple 5 lug options. Hope this helps.
Dave
