361 Woes

I'd guess the intake valve might be stuck for some reason. Before doing any more work, I'd check to see if that's not the reason. Assemble the valve train and turn the motor over by hand watching the valve in question for "free" movement when its supposed to open.

Valves get stuck, or can get stuck shortly after being started when engines sit for years and years.

If the valve is stuck, I'd pull the heads and do a basic clean up and overhaul. You may have another valve getting ready to pull the same stunt. I encourage you to replace all the exhaust studs if you pull the heads, regardless of their appearance, they get old and then snap when you reinstall them.