rotor phasing ain't affected by initial or mechanical timeing, it is set in stone & changed only by vac adv. As you know RP is where the rotor is clocked at when it fires & if is is impossible to set the housing to that firing point then drill a 1/2" hole in the top flat area of the dist cap 2/3 of the way inbetween the center cap terminal and the #1 cap terminal (2/3 closer to #1) & start the eng & idle & shine your timing light straight down in the hole to freeze the rotor position just like you are setting timing & see if the rotor is pointing at or nearly at the cap terminal. vac adv (if used & I would) will shift it CW on a BB when you plug it in to manifold to activate it. use a junk cap if you dont want to drill yours but it does help evacuate ionized air molecules which can promote crossfire. EDIT from reading the above posts it for sure could be an air gap or other Pertronix specific issue. Holler how it turns out.