On the 50’s and early 60’s OMC tanks I have found my el cheapo blow gun screws right on to the tank valve. I then use teflon tape to seal the filler cap threads. Be careful as it is easy to inflate the tanks like a balloon. go slow and possibly use some backing on the areas that you don’t want to blow out.

personally, I would not recommend compressed air, even tho it has been the norm since the beginning of time.
I lean more towards filling with water and using a hand pump to control the internal pressure.
just like pressure testing a scuba tank – you do not want 80cf of air at 3,000 psi exploding all at once. (should failure occur).
auto repair shops use this technique with testing radiators for leaks…….. with water – not compressed air.
some bicycle pumps can reach over 100psi. a good recommendation would be the 12-volt air compressor from Harbor Freight.
it pumps slow but steady pressure up to 100 or 150psi.

if the gas tank ruptures when filled with water, very little chance you will be hurt .
if the tank ruptures when filled with compressed air, very good chance you could get hurt – badly.
the tire valve can still be used to connect a hand pump to after filling with water then pressureize it accordingly.

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Cleaning Gram’s basement I found a stash of these. When I read this thread I thought of you guys that work on gas tanks, lower units, and
headers. These may be useful for pressure testing. I also remember an old thread where one of these valves was soldered into a spark
plug base and used to free up stuck powerheads. I have a bunch of each kind along with many fittings and connectors. I’ll sell them
cheap if there is interest.