The rear retard vacuum line never worked well as an emmisions device in the first place. My recommendation is to plug the rear line and only use the front advance line on the dizzy. For appearance purposes, I'm using the dual vacuum port as from what I can tell, the 351W manual cars with AC received them. I hooked up the lines per the factory routing, but I plugged the rear line to make it non functional.
I'm not familiar with the routing of the vacuum lines on a I6, but on a V8, the bottom port of the vacuum switch goes to manifold vacuum, the middle port goes to the dizzy (front advance port) and the top port goes to the carb's ported vacuum.
408 Windsor, port matched Torker II, 735 cfm 428CJ carb, Comp Cams Hyd Roller, AFR 205s, 1 3/4" Accufab headers, 10.1 to 1 SRPs, Centerforce Dual fr clutch, Tremec TKO, 31 spline 9" w/3.70 Trac Lock, 3" chrome moly drive shaft, Comp Eng slider traction bars, sliders on rear leafs and 2.5" Magnaflow stainless exhaust.