Help - Toilet Tank Leaking

I am trying to get my bathroom renovation complete before the one year mark, but the toilet is not cooperating! I have replaced all the seals at least once and I have finally narrowed down the leak to the connection of the overflow tube to the tank.

With the tank off, I put a quart of water in the tank and I can see the water seeping from around the nut holding the overflow/flapper assembly. I have replaced the seal and tightened the nut as much as I dare. Can I put plumbers putty or silicone or something around the seal?

Plumbers Putty? NO NO NO
Silicone, Just No

Remove the flush valve and sand the inside of the tank, checking for gaps and cracks.
Reassemble