I did mine, and it's been ok for several years, now.  There was a place in
the bottom where the head of a mounting bolt  had pecked a hole in the
bottom of the tank.  I made a patch from a thin piece of brass for that.
 One of the biggest problems was the mounting cradle, which is steel, was
originally spot welded to the tank, which is brass.  A previous "repair",
(or vandalism), had involved ripping it loose; tearing holes in the tank.
They opened up while I was soldering the patch in place.  Then, seams
started leaking...
  I made a plate, to seal the thermostat flange; so I could pressure test
the brass tank in a tub of water without the cradle.   Then I added the
cradle.  It took several tries to get it leak proof.  I tinned the areas
around the holes drilled in the cradle, on the tank and cradle; and soldered
it in place with as little heat as possible, one hole at a time.
 I wish I had modified the tank a little, to give more clearance at the
fitting for the overflow tube, on the filler neck.  As is, I have to use
thin wall tube to the recovery tank I added.
 When you bolt the tank in place, you have to be careful not to put strain
on it when you tighten the mounting bracket. I hear there is a thermostat
gasket made for Chevy's that will seal eroded flanges, if it's been leaking
there since the thermostat was changed in '65.  I cleaned the flange, filled
it with epoxy, and sanded it smooth.
Mike M.

Mine was bad too, and when I had the Avanti restored I had them re build the
tank.  They have a radiator rebuild shop there too.  It looks like new after
5 years and holds water well.  Old Coachworks in Yorkville, IL.  Highly
recommended.
Dick S.
63R-1028 (with the rear window still intact)
61 T-Cab (for sale BTW)
49 2R5