quick update.. Decided to pop the original backbox on, and install hangers from a spare high speed playfield onto my playfield and see how it looks in the cabinet.
Marked the positions, everything lines up... but the shooter rod position is too low, it doesn't fit underneath. My only guess is that because the lockbar mech is from a 1978 pin that perhaps everything was higher back then (coming just out of the EM era with very flat playfields). So it seems I have 2 options:
1. Attempt to lower the mech. It does seem higher than usual, but I'm lining it up with the bolt that holds the coin door. If I can drill a new hole, maybe this is feasible
2. Make custom hangers that lower the playfield. Not something I really want to do, those hangers are pretty thick steel, and both of them have to match otherwise the playfield won't be level.
I will say, it IS nice seeing the playfield inside a finished cabinet. I think it'll help motivate me to keep going.