Just had the carb professionally rebuilt, so I'm thinking that its something else.
In a previous thread Uncle Dan told you the '74 vintage 1945 was a crappy carb that had numerous problems....
Did your professional carb rebuilder know all about how bad those carbs were and did he know how to fix them? Cause gaskets, and a soak in the ultrasonic cleaner won't fix a bad carb. I'm not saying your carb is bad, but I am asking why you had it rebuilt?
This is not intended to sound unkind, but if you don't do the basic diagonstics you can't find the problem..you are making progress with the vacuum gauge.. and a bouncing gauge is telling you something.. A compression test is also useful... but tell us more about the vacuum gauge. It is confirming what you are unhappy about, but we need more information to determine What/why is the issue..
WHY is the vacuum gauge bouncing? Is the next question.. Because there is a variation in the intake manifold that the vacuum gauge is indicating.
Tell us more about how often it bounces? Is it a constant bounce, an occasional bounce. An occasional repeated bounce indicates one cyclinder. A constant bounce is all the cylinders... which brings up the next question:
When was the last time you adjusted the valves?
As for doing an engine swap, I suggest NOT... just sell the car and buy what you want to be driving.. OR get a shop to rebuild slant 6 in the car if necessary..
An engine swap is very involved.. countless things much more difficult than the topic at hand is involved.. YES It is something you can do.. but it is involved, it is expensive, and if the present issue is vexing, just wait until you spend the thousands, and have a car with a brand new engine that vibrates going down the road...