There is an easier solution. GM part# 10033125 is a small pressure valve that goes on the end of your vent line. Simply route the line outside and attach this small approx 1"x 2" valve. I run a orig 62 Rambler tank, in tank fuel pump and sump, stock cap as it is part of body, and this valve. When pressure builds that valve simply lets it out and then shuts again. A simple spring daiphram set up I believe.
Of course it has to be routed outside of the trunk area. Also if you have a trap door to access that tank make sure it is sealed with a gasket when it closes.
You should ask them exactly how they want that tank vented as you might collapse it if isn't braced but I am sure it is.
Good luck.
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