Airforce texan .510 cal. What's the twist rate in barrel??

I 've been trying to find out what twist is in my 50 cal texan? I've shot up to 500 grain cast ammo in my gun no problem. I'm going to upgrade my texan with the African ordinance 4500 psi valve and wanted to buy a new cast mold for a 625 grain boat tail. Just what to make sure the 625 grain that are 1.375 in long will stabilize. Thanks