340 camshaft break-in. Well.....almost

Was the gas cap off?
Sometimes they clog or aren't the correct vented type.
Other thing that can happen is the pickup tube on the sender gets clogged full of old gas gum...or a rodent or leaves from an open fill neck.
Try the fuel can with hose up under k member to fuel pump trick. Maybe the pump isn't bad?

Purists might cringe... but you could trim the cover with tin snips.