Vapour Lock Issue

Since you're running a mechanical pump, This is what I'd do; I'd run the fuel line direct to the carb, and retest.
Here's my thinking; At 60 mph, your fuel requirement is really low.I think the regulator is unable to properly regulate at that low a setting, and eventually the bowl runs dry.
Either that or the pump is sucking air. Could be anywhere on the suction side, all the way to the sender.
And here's the thinking on that; the usual culprit is the rubber line connecting the sender to the main line, at the back. It's high enough to not leak liquid. And as long as the fuel requirement is high, the pump gets enough fuel. On a bypass system some or most of the air will get returned. That's why I recommended a test for that.
But at low fuel requirements, the pump draws mostly air, and sooner or la.....
Now, the line may be new, as stated, but if the line to sender is not secure, this is how the trouble starts. If using a metallic main line, I double gear-clamp those,180* apart. If using AN fittings, you're on your own, cuz I don't use that stuff.I also don't put filters on the suction side, without double clamping.
When single clamping, the rubber hose tends to bunch up right at the screw. I've had them leak there. Double clamping, 180*apart, eliminates that possibility.