oh sorry.. my tubes were aluminum so not really relevant data.. .750 od , 500 id, 9/16-18 threaded ends , small volume.. 6061.. most recent steel tube I did was similar to yours , .866 od, .745 i.d , with M20 threaded cap.. 2000 psi max fill , and unknown Chinese steel. So your design would actually be safer by default and withstanding less pressure on the end cap