There is ample proof in the Bible itself that Jesus died on a "Christian" cross and not on a crux simplex (in deference to Ian I hasten to add, assmuing Jesus existed ).
(1) The length of time required for mortality for a person executed in the manner reportedly used with Jesus required the breaking of the legs of the other two men being executed, which meant they were still alive. As shown in Mary's analysis, this would have been impossible for a person sound in body after 30 minutes on a crux simplex. The fact that the soldiers intended to have to do this to hasten death shows that they expected the condemned to still be alive.
(2) Thomas, who would have known exactly what the instrument of execution looked like, twice used the plural form "nails" when referring to the print in the hands of Jesus. A crux simplex only required the use of one nail through both hands. Given the size of the nails in question, it is unreasonable to imagine two nails through crossed hands above the head. There simply isn't room, even with fairly large hands.
But the funniest part of it to me is the JW retort regarding the pagan-ness of the symbology in the "Christian" cross. The upright pole is a ubiquitous symbol of male phalli and male fertility throughout pagan civilizations. I am not aware of a single exception to that rule, whatever other symbols might also be present in the pagan culture to represent male fertility. Even in modern-day hedonistic milieux, like erotica clubs, one finds the upright pole being used symbolically as a phallus. It is certainly the oldest symbolic representation of male fertility.
The thought that a pagan culture (one that delighted in both physical and psychological torments for convicted criminals) would not use pagan symbology in its executions is laughable.