Fuel filler / Rollover valve / etc ??

[yossarian19]

So, I'm going to be running an LS1 pump in a Mustang tank under my '59.
I see tanks listed as being with rollover valves, and I know they are factory equipment in most cars, but I have to ask: what is it for, and do I need one?

Also - modern filler pipes come with vent lines that run between the gas cap & tank in parallel with the filler tube itself. Is this just to let air purge out as you fill the tank, or is there some other purpose I need to keep in mind?

As it is, I have a return line & a pressure line w/ filter set up & the Mustang's 45 degree 1.5" or so filler neck. No other plumbing.
What's needed?