Hate to start another thread ... but.. help !

Next time you measure 4 V at coil+ (to gnd) when running, measure the key side of the ballast to ground. It should be the same as BATT+ within ~0.5 V. If much lower, keep going upstream until you find the culprit. Start at the key switch IGN terminal. Make sure your ground in the cabin is the same as your engine ground (less than 0.5 V drop). Of course, across the battery should measure 12.6 V or higher when running.

If the voltage is OK on the key side of the ballast, then you have too much drop thru the ballast or wiring to coil+. There are many different ballasts and coils and they don't all play well together. Too confusing for me. I just run HEI with no ballast in all my cars but my Newport (planned). For your 360, you can get an HEI distributor and E-core coil for ~$70 on ebay and kiss ballasts goodby. Look up "ready to run" and "distributor". I got mine from stevewhite.