Not sure what you have going on with that wiring. The voltage regulator looks older, you may want to replace it with a modern solid state one. That being said, newer parts these day are not always better than older ones that are still working well. Do not buy the reproduction early style voltage regulators, they are junk.

My brother had a similar problem to yours years ago and it wound up being the ignition switch. Running a 12 volt source to the coil should have eliminated that as a potential problem though.

Modern ECU replacements leave a lot to be desired. Jegs sells a decent replacement. Their orange box is good to about 5,500 rpm and their blue box is essentially the old chrome box. Bill Evans can also sell you a reproduction engine harness that is made for electronic ignition.

MSD blaster II coils are not recommended for use with stock style ECUs. They are also not meant to be mounted horizontally. I would replace the coil if you buy a new ECU. Bad grounds are also a common issue with ECUs. I have a ground strap from the back of my block to the ECU as an added fail safe.

When you check for voltage, always jiggle the wires a bit and see if the meter changes. We’re talking about nearly sixty year old wiring here, might detect a broken wire or bad crimp connection.