Ignition problem: starts and runs in neutral but dies off in park

1973 Duster - original wiring, lots of brittle old wires. Rebuilt 360 with 904.

Pre-engine swap problem: wouldn't always start in Park, but would start in Neutral.

I swapped in the 360 I built, which ran fine on the run stand with an Accel coil and Skip White distributor. When I put it into the car, I used the old distributor and coil, with pigtails and OEM ECU (from the 318 that was in there). Ran kind of rough but got me home. At first I was worried that the Comp Cams 268 may have flattened a lobe, but I took off the valve cover and bumped the engine over to see that all the rockers are doing their thing.

In the process of installing an HEI setup, I ran into all sorts of weird problems. Among other things, I noticed a voltage drop of at least 0.5 volts coming out of the ballast resistor wires as I was jumping them (used spade terminals and wires to connect).

So I put back the stock distributor and coil and now it will only start and run in Neutral; it dies fairly quickly if put into gear and if put into Park, it kind of winds down and then dies.

Any hints as to where I should start hunting? More information needed?