What is Transmission mount bolt size?

['65 chevy lover]

I'm having a difficult time finding the correct bolt size for the rear mount of my TH400. Not sure what year model it is. Bought it at Bishops International. It didn't have bolts in it. I've tried 3/8 and 7/16...neither worked. I'm wondering if they might be metric. From what I can tell, they appear to be fine thread. I don't want to screw up the threads trying to put a boult in there that's the wrong size/thread count. Anyone know what size and thread count?

Thanks