1973 340 issues.

If you can work a hand valve of some sort to manage the air, that would help. You want to test that dead cylinder with the valves closed; i.e., up on the compression stroke close to the time for it to fire. You can use the distributor as a reference by pulling the cap off, and lining up the rotor with the #2 spark tower on the cap.

Good luck on that thumb! Ow...