Next time you have the trans out, check the runnout of the crank flange using a good dial indicator. Measure for runnout on the flat surface the flywheel sits on and then turn the indicator 90 degrees and check the runnout of the pilot that extents through the middle of the flywheel. You shouldn't see more than about .003" runnout. While you are at it, measure crank end play to make sure your thrust bearing is OK.
Another problem that "could" cause multiple flywheel failures: If the block has been align bored, the main bore moves up in relation to the centerline of the transmission. Normally the bore moves only .005" and should not cause a major problem. But if the machinist made a bigger move in the main bore to correct bigger problems, the misalignment between the engine and trans can cause this type of problem. Aftermarket adjustable trans alignment pins can be used to correct this problem.
If that looks OK, then the T convertor may be the problem.
Dave