for what its worth
i don't know what pipe nipple size you chose to use, but
i can't remember which it is, but mid size trucks and some heavy trucks use a brass cap that fits either a
1.5 or 2 inch pipe nipple,, iirc it is a 2"
the upside to useing one is they have lead plugs in them that allow for venting in case of a fire, they melt out
and keep things from really getting exciting
they also make vent units that screw into a 1/4 inch pipe thread hole, that are anti tip in that if the tank rolls over it won't leak
but in normal operation will vent and keep the internal pressure normalized,, they are pretty inexpensive
if you are going to use the iron cap, i would suggest drilling a couple of holes in it and then refilling the holes with lead as
a safety, or you could chamfer the holes on each side and hammer a piece of lead into them to provide sealing and also allow them to
blow out if there is a pressure buildup.
in any case if you have a violent explosion within the tank, no safety cap or vent is going to save you in my opinion
inspirational project though, looks good
folks like you, make me want to get off my dead ass and get to assembling!
bob g