I installed a Van's tank in my '63 this summer. I assume it is very similar to 64-66. So far, I haven't had any real problems with it. I re-used my original filler neck, and it seems like maybe it doesn't align perfectly, but it doesn't leak, either. That's probably user error on my part. Also, the last weld for the vent tube was broken when it arrived, but that doesn't affect functionality at all. It looked like maybe it got bumped in shipping?
Some small tips:
1. Get a tank flange tool, it will make removing the pickup easier, especially if you ever do it while the tank is still in the car. I found
this Ford one was cheaper, but fits perfectly and even said "Ford/Mopar" on the package
2. If you use the flange tool to re-install the pickup, don't over-torque it! You'll bend the locator tabs and then it
will leak
3. The little piece of rubber tube that connects the vent tube on the tank to the tube on the filler neck is super cheap. Mine ripped almost instantly. I replaced it with some 3/8 rubber fuel line and hose clamps, seems much more sturdy
4. For the pickup and the filler neck, I got some Permatex gasket dressing that specifically said it was gasoline resistant. Not sure if it helped, but nothing is leaking.
5. Once you have the new tank in, replace your fuel filter soon after to get rid of anything you stirred up during removal and install
6. Protect your work by adding an ethanol treatment, like Stabil 360, or use "rec gas" if that's available near you
7. When you do the first fill up, I
HIGHLY recommend filling your tank at home, from cans, a few gallons at a time. Have someone watch under the car for any drips or leaks. That way you can catch and resolve any problems quickly, instead of trying to figure out how to get home alive with a leaking gas tank, and winding up on your local gas station's "banned" list.
8. For extra security (after learning the hard way) I jacked up the car from the rear, so that the fuel would shift forward, toward the sender. That way, I reasoned if it started to leak I could just set the car back down level and the leak would stop.
9. If you need to drain the tank, there's a drain plug on the bottom of the tank, near the diff. I couldn't find mine at first because there was so much undercoating. Just be careful you don't tear the gasket if you plan on re-using it.