I've been thru this before...Man! What a pain! The mild 350 in my work truck LOVED the taste of a freshly cooked starter. Ate one every 2-3 months. The parts store voided my lifetime warranty! I installed a heat shield, a new flexplate, heavier battery terminals, and a new battery; nothing helped. This went on for over a year.
Finally, while disconnecting the battery for some unrelated maintenance I noticed that the wire leading from the positive terminal to the terminal block on the core support was gacked. It had maybe 2 strands of copper holding it on the terminal. I replaced that one 12" piece of wire and fixed the problem! My truck hasn't ate a starter since (almost a year now).
(Sorry for the long story, but I DO sympathise. Maybe this'll help you?) Bryan