I have a 99 ECLB K2500 w/8600 lb JB7 brakes (2.5”) drum brakes on the 14 bolt 9.5 SF rear axle w/o Gov lock (G80) & 3.73 (GT4), I have the 5.7L, and 4L80 also.
I believe the deciding factor on FF was powertrain combos. In the K2500, 5.7L got the SF, you needed either the 6.5 TD or 7.4L gas...