By vent on top you mean on the front/top of the tank facing forward, correct? If so that's the vent you want to use. That vent should hook to a hard line which goes to the passenger fender in the engine bay and then to the charcoal canister.
If your fuel sending unit has an additional port it's not a vent, it's for a fuel return line. You can plug that by connecting a short piece of fuel line to it and putting a plug in the other end of the short fuel line, such as a short bolt. Make sure the plug is securely clamped to the line.
Furthermore just some additional info in case you aren't familiar with aftermarket sending units. Aftermarket sending units have a few things to be aware of.
First before installing the tank into the car install the aftermarket sending unit. Be sure that the pickup tube goes all the way to the bottom of the tank. Some aftermarket sending units don't reach the bottom of the tank, or are bent too far down which will affect the seal. You can look through the filler neck hole with a flashlight to see the positioning of the pickup tube
Test for leaks at the sending unit before installing. You'll most likely need to use your OEM sending unit lock ring if you still have it.
Aftermarket sending units are linear and your stock fuel gauge isn't. If you intend to use the stock fuel gauge you will need to get used to the changes, or install a Meter Match. Search this forum for more info on Meter Match.
