Ecu locations

so I'm thinking about where to mount the ECU In a 63 valiant.

My first thought was to "hide" the ecu to keep a more vintage look. I thought of mounting it under the battery tray. Then I thought of just putting it out there and mounting it on the left inner fender just like the 73+ cars.

Now I'm thinking of mounting it under the dash and running the wires through the firewall through a grommet.

Is mounting a ecu under the dash a bad idea?? Is there any reason it wouldn't work from there, I'm thinking being the glove box or somewhere near there.