Turning the ferrous metal cap against the rust on the threads may result in a...
You are assuming what it holding the lid tight is rust. It may not be rust.
Black powder is more "age stable" than smokeless as it doesn't chemically break down over time the way smokeless does.
Black powder can keep its full strength and ability (including it sensitivity) for well over a century (and who knows how much longer?) if it is properly stored.
SO assume the powder is full strength and possibly even "grouchier" than when new. Spark, open flame, COMPRESSION and IMPACT have been known to set off black powder. STATIC ELECTRICITY has done it. And, powder has also detonated for no known reason.
35+ years in a garage, it might be rust in the threads, or it might be something else. What other chemical vapors might the can have been exposed to during those years? Some chemical vapors will leave crystalline deposits. Sometimes those deposits are unstable and can themselves detonate. All depends on exact chemicals and conditions.
My experience with model paint jars and stuck lids is that metal tools (pliers) on the metal caps seldom have the grip required. The jaws "slip". Using a thin piece of soft rubber gives the jaws a better grip.
HEAT often works well, to loosen the caps, applying it using HOT WATER, NOT any kind of flame, OR heat gun or anything like that
Pouring very hot water over the cap (slowly), or soaking just the cap in hot water could cause it to expand just enough to break the bond sealing it, and allow it to be unscrewed.
If it is just rust, inverting the can and using a penetrating oil such as Liquid Wrench or Kroil and allowing it some time to soak the threads might do the trick, as well.
Or you can just write the whole thing off as an unnecessary risk and call the professionals (Fire Dept, usually) on how to safely dispose of the hazardous (explosive) content. You might also check with the Police Bomb Squad for their input. The bomb squad tends to be very "reactive" so I'd talk to hazmat disposal people first. IF they say call the bomb squad, then do so.
And then be prepared for your entire city block to be evacuated and coverage on the nightly news....
Good Luck!