It is a 489 case with 3.91 gears that I purchased from Dr. Diff. Unfortunately I don't know what type of sure grip it is.

Also, do I need to drain the gear oil out of the 3rd member and then put in the friction additive and then put new gear oil back in?

70 Cuda 340 4 speed - now stroked to 416ci (SOLD)
2017 Mustang Shelby GT350