Well I have almost 5000 miles on the transmission, I did have it pop out of 1st gear twice going down a grade, and a few times out of 5th. The 5th gear issue turned out to be a driveshaft issue, pinion angle was off a couple of degrees.
The car is heavy, 4100 lbs, 572 hemi, 735 HP, 717 T, 4:30 dana 60, 30" rear tires.
I think the transmission is great, just sayin

1967 GTX 572 HEMI 735 HP, 717 FT Lb T, , A-855 5 Speed, Dana 60 4:30 Gears, 30" Rear Tires