Do I need to seal or prep the inside before filling it with fuel?

Aluminum fuel cells should be clean and dry before use. If this is a new tank, rinse the interior thoroughly and allow it to air dry completely before connecting fuel lines and filling.

What fuel line size does the bottom feed outlet accept?

Outlet diameter is not specified in the product data. Measure or test-fit your existing fuel line to confirm compatibility, or contact supplier with the tank serial number for outlet specs.

Can this tank be mounted horizontally, or does it need to stay upright?

Bottom feed tanks work best in a level or slightly tilted orientation to keep the pickup submerged. Horizontal mounting is possible depending on your mounting location; verify clearance and fuel sump depth before final installation.