Robin,
I think you need to go back an read the Safe loades and cannon plans post. The 2 ozs. per inch rule is for tubes over 2 inches. Below is the section on under 2 inch guns. Your maximum safe load sould have 330 grains.
You projectile weight was a bit excessive also. That gun was probably for round ball and round iron ball that size would weight 5 1/3 oz. Even round lead ball would have only weighted 8 oz or so.
You definitely proofed that gun. It's a wonder you you didin't break some thing more than the vent. To fix the vent just drill it out and install a vent liner.
Moderator note: These rules applies only to guns over 2 inch bore. For guns smaller than 2 inch
THE MORE COMPLETE CANNONEER compiled Agreeably to the Regulations of the War Department as published in "Artillery Drill" by George Patten, 1861 and Containing Other Observations on Antique Cannon By M.C. Switlik with selected excerts from other artillery manuals Appendix I has a load chart which is posted her by permission of the author.
This load chart is maximum load of a unpatched ball attached to a sabot.
Addendum For mortars and howitzers the the diameter of the powder chamber is the considered the bore diameter.
Click on this link to be taken to the post on loading cannon cartridges
Making Cannon Cartridges