361 Woes

Unless I am mistaken, the heads on a '66 361 will have closed chambers, meaning if you were to swap later model open chamber heads on to it - you will loose compression ratio by probably close to 1 point - ie 9:1 to 8:1.

Try a compression or leak down test yet ??

Why not pull the valve covers and inspect them to see if the stem seals are still there. Just a thought. If they are gone then consider changing the seals.

It can be done without pulling the heads, backyard mechanic style. Ive seen a lever type tool sold that uses the rocker shaft as a fulcrum to pop the spring retainer apart. You would either need to pressurize one cylinder at a time with compressed air to hold the valve shut - or Ive heard of stuffing cotton clothesline into the cylinder one at a time at BDC thru the plug hole, and then rotate the crank as far as possible to TDC for the cylinder in question, which would also shut the valves.

Good luck with it.