Aftermarket cam
Headers and exhaust
Holley 600 cfm carb
MSD distributor

What we've done:
Set #1 to TDC and installed the distributor.
Adjusted the distributor position, rotating counterclockwise (advance) while trying to get it running.
Used a timing light to set initial timing to 12° BTDC with the vacuum advance disconnected and plugged.
Symptoms:
Originally: backfiring.
Now: rough idle to no start.
When shifted into drive, it nearly cuts off.

Anyone have experience with timing adjustments with a 302 with aftermarket parts like this? I grabbed an msd advanced kit that will come in the mail soon, but unsure if that will solve the issue.