Did anyone happen to see this yet?
https://ridetech.com/product/gm-afx-hub-spindles/
I went with both upper and lower SPC arms on my 68 FB and just upper SPC arms on my 64 Tempest. I can't really say I love them. They have their pros and cons.
The SPC lower BJ is 1 deg forward to allow more caster. I use to have some tire rub on one fender. Now I have no rub and 5 deg caster dialed in. MY 68 is a mis-match of parts but it handles pretty good now considering its on 235/60/15 tires.
When SC&C was still around Marcus always suggested going with the SPC upper arms and keep the stock lower arms. The upper arms have the tall BJ and the adjustability to increase caster, so this makes sense for a mostly street driven car.
The SPC lower arms have a lowered spring pocket. I had to use several shims to make them work with my 2" lower Ridetech springs.