regarding altitude differences, 91.217 requires that the altimeter report pressure altitude correctly within 125 feet. So two aircraft, one reporting FL050 but 125ft high at 5,125 MSL and another reporting FL053, but 125ft low at 5,175 MSL will pass very close to each other, especially if one of them might be an ASH-30 in a level turn.

https://www.law.cornell.edu/cfr/text/14/91.217