I have a funny feeling the Model J just happened to go bad while you were working on the dash (I've seen it before)

I would do the following:
Bleed the line from the propane bottle to the vaporizer (bleed it away from you- it's very cold & EXTREMELY flammable- make sure no open flames & plenty of ventilation for fumes to dissipate) & hook line back up to bottle

Start engine-if engine fails to start- just quickly crack open & close the propane bottle shut off valve.

Once engine started- let engine begin to run out of gas- as engine begins to stall- crack open (very quickly) & re-close the shut off valve once again. Engine should pick up & run normal once again.

If you can keep the engine running doing this procedure then the Model J regulator is faulty (passing to much fuel & flooding the engine). Replace with the newer Cobra regulator.