How much gain with lower gears?

I recently blew up the 3.58s (early 60s case) after driving to Lebanon Valley for Musclepalooza 130 miles....and changed to 3.91 gears. Car ran 13.1 @ 104 and change put in 3.91s hoping to get a 12. Didn't happen. Torque converter now stalls at a lower rpm. I did change the cam btw but the mph only increased by a little. Bottom line...gear change did not do what I wanted and caused my converter to stall at a lower rpm hurting my et. Put a 3500 Dynamic converter in this weekend past. Lebanon Valley this Sunday for Musclepalooza. 12s here we come!