3/8". You want the feed lines (pressure lines) all 3/8", only the return line from the regulator to the tank is 5/16" (though it could also be 3/8").

Thank you!

Answers embedded above. As Jody said above the return line could also be 3/8. Whatever is easier.

Here are a few options that you can do.

Use this regulator. Has everything you need to hookup the lines.
https://www.summitracing.com/parts/tnk-ls9904-kit

Buy three premade 6AN lines. Something like these, depending on the length that you need. https://www.summitracing.com/parts/fra-60261216bl

Your Holley tank hat output should be 1/4NPT. You need two adapters from 1/4 to 6an. https://www.summitracing.com/parts/sum-220647b

Check your Holley fuel rails for the size of the AN fittings used for your feed line. If its 8AN get an 6AN adapter instead.

Don’t use that FPR! Read the reviews. AC Delco or Wix only. The rest are Chinesium garbage.

Also don’t use those push on connectors with plastic parts. Get the threaded ones.

Thanks!

Awesome thank you!

Thanks, I always try to stick with AC Delco if I can.