The vacuum 100% should be as you understand it Louie. It should be in the green at idle and return there when the engine reaches higher RPMs and there is no load.
You have mentioned over and over that the timing chain could be off. Now that you elaborated, I get why you keep mentioning that.
The exhaust being restricted can and will produce low vacuum. Less air out means a back flow of air in the "air pump" (engine). There are other things that can do the same thing..... but.... when you parked it the exhaust was not restricted, etc. This is something that you laid hands on and often times during a big project, a NEW problem is a result of something that we laid hands on.
At WOT, I could feel air pushing into my face but it was not backfiring.
I am going to go back to this. This was a big clue for me.
There are a few times that I have seen this. One of the older GM 4 cyl engines had a timing chain. 2.0L pushrod engine IIRC but it does not matter. They used a timing chain tensioner/guide system that had a plastic guide on them that rode against the chain. When the plastic piece would break off the guide, my theory was that it traveled up and around the timing gear and move it AHEAD a tooth. Yes, when you tore them down with this symptom the camshaft would be advanced and not retarded as most timing chain and out of time failures occur. I found several of these engines to be the same way. The cam was advanced, not retarded. To back up that this can happen, I learned a trick from a guy about 5 or so years ago on an particular Hyndai engine that we did timing belt services on regularly. To get them together in time just sucked. You would take them apart over and over in a cramped place. Very difficult and came out wrong more than right. He showed me that you could stick a stubby phillips screw driver in a tooth and roll the engine one way or the other and advance or retard a cam by exactly one tooth by letting the screw driver enter in the tooth and roll all the way through the rotation that it took for the screwdriver to pass under the timing belt and the sprocket one time. I do not know if this makes sense or not, but it works every time. The reason I mention it is that it explains my older findings on the 2.0L engine.
Another couple of times I have seen it on some performance builds where guys learn the degreeing a cam (either advancing it or retarding it a few degrees) can increase or decrease where an engines max torque and HP will occur. It can alter if it digs more out of the hole and has more high RPM grunt. Rather than spend the big bucks on a timing gear set that is meant to make the adjustments, they slap the chain on a full tooth off. Once again, the ones that I have seen that blow back through the carb are advanced.
Yes, I think that you pulling your timing cover and checking this is your best next move. It is what I would do for sure.
I am betting that your camshaft is advanced. The exhaust cannot get out of the engine fast enough. It is leaving cylinder pressure that backs back up into the intake when the intake valves open.
Thanks for buying a vacuum gauge. That really helped. Your engine vacuum is a very basic thing. This IS the rabbit hole to go down.