While the 7jp4 is rated for I think 5-6 kv the 3bp7 is rated for 2 kv so I'm wondering what would happen if I plugged the 3b into a working 7inch set with the higher difference in hv, would it work or would the gun arc inside and I guess there would be verticle and horizontal issues as well.

Worst case scenario: arcing along dag on inside or outside surface of glass causes it to implode.

Best case scenario: it's so bright the phosphors start to burn.