I'm not sure if this is 100% correct for '68, but I'm trying to help here. I know in '69 the AC line that comes out of the top of the box is different for SB and BB. The SB line curves toward the fender (to the outside). The BB curves toward the engine (to the inside). If someone has a '68 BB with air, they could check this, but I have two '68s and one '69 with SB and factory air, and they curve to the fender. The line I am talking about has the POA valve on it. Hope this helps. Please don't bash me if I'm wrong, I'm just trying to help. Take care.
69 SSRS Frame-off Resto
81 Z-28 377ci Drag Car