I found some more info at NHRA.com...
NHRA finalizes 1999 rules changes
9-7-98
INDIANAPOLIS - The NHRA General Rules Committee
finalized rule changes for the 1999 racing season at a meeting Wednesday night during the U.S. Nationals. Details on some rule changes are still being ironed out, but highlights of the changes are as follows.
E.T. Brackets
A long-standing rule that requires any car with
independent rear suspension (IRS), weighing over 2000 pounds, and running 10.99 or quicker to replace the IRS with a conventional rear-end housing has been modified.
For 1999, only those IRS designs which utilize a lower control arm only (like a 1963 to 1982 Corvette) will have to comply with the above requirement. If the suspension utilizes both upper and lower control arms, the IRS may be retained regardless of weight or e.t.
Don