#4323 3/8-16 N.C. x 1" hex bolt is what's listed.
Be sure and shim the gap, if it's not a flush fit between trans. boss to the frame ("5th") mount. If the trans boss fits flush, make sure the trans is sitting completely down in the base plate. check all the way around with .004" feeler gauge. A gap anywhere in the mounting will eventually crack the frame, or maybe the case, if it's the 5th mount that's gapped over 0.004".