[taff2]

Looking at the dark areas in the crack it does appear to have started cracking a while back. Definitely got some cap fretting,what are you revving it to? And why are there main studs on only number 2 and 4 mains?
You could also do with modifying the thrust bearing, file a small chamfer along the parting lines of the shells- about 0.020" will do it. Have you restricted the oil flow to the converter/cooling circuit in the TH400? You sound like you have a recurring problem with the thrust bearing and modified TH400's are notorious for causing engine thrust bearing wear. Increased trans line pressure translates into higher cooler circuit pressure which can force the converter forward,causing thrust bearing problems.