Westinghouse, then Power X, use to have a post mortum of photographs in their Power Semiconductor Handbook. Its probably out of print. IR had one too. Check with the power semiconductor folks I'm sure they have references. Typically failure at the edges were caused by either voltage or lack...
Check with the manfacturers it all depends on the conditions of usage. The insulation type may meet the xpl basic requirements at a given voltage but, flexability, extremes in temperature and surrounding atmosphere may drive you toward one type or another. Weight may also be a factor. The same...
Your 3ph formula is correct for the basic approximation but, check the NEC for overload and loading factors. I'd suspect a 750kVA xfmr is closer to the choice. Don't forget harmonics and their effects. Also future expansion or loading should be considered. I'm sure the power company will work...