Anyone put a fuel cell in a 64-66 Cuda? Bad idea? Fumes?

Bad idea. Don't run a cell. There is absolutely no reason to.

You can hook the factory fill to the cell and vent the cell under the car. That would eliminate any smell but you'll still lose your trunk.

Here are 2 viable options and you can still keep your trunk.

1. Hot rod city garage makes aluminum fuel cells that bolt in the factory location. They can be setup for EFI, etc.

2. Aeromotive makes stealth tanks and EFI pumps that work in them. These require a bit more fabrication work to install but have some other advantages.