DYNO disaster ? Help

I would ditch that orange box, my car never ran right with one of those things. You can go to local parts store, ask for ign module for '73 Dart 318, and get something that works better then that. Or go with the Mopar chrome box. Also make sure ign box is grounded well. When I used to run one I ran a dedicated wire from ign box to engine just to be sure.

Make sure bulk head connector on the fire wall is free of corrosion. I like to pull them off, spray Electrical Parts Cleaner in there, put some dielectric grease in, and put them back together. Look under dash and make sure all connectors are pushed in tight into the bulk head connector.

I would do a compression test like Moper said that would give some idea what compression is. I would find out what pistons and what head gaskets were used if you can.