I have observed the Vacuum tank "emptying" ( filling?).
This happens often on C-3's with the large storage tank / crash bar.
Takes a few minutes to get back to where it needs to be, longer if little leaks, then fine.
In life, I have learned to not look a gift horse in the mouth.
Drive it and enjoy it.
If the problem returns, we will deal with it.
p.s
Keep the key in the start position a bit longer than other cars, even up to 10-12 seconds.
Most backfires occur right as you release the key and the engine was beginning to start.
These LT-5's have a few idiosyncrasies.
Marty