Vapour Lock Issue

I think we're beginning to see the story.

1;The issue only occurs when I sit at 60mph on the highway, it just coughs and dies.
I though the bypass reg and return line had fixed it but not so....although it has helped compared to the deadhead reg.
6;It will not start straight up again, needs to crank for a fair while before it will start.
Also I have a 1" plastic style spacer. (sorry forgot to mention both those points).It takes a good 30mins of driving before it will play up so I assume it is the heat being the problem.
12;The fuel tank was removed and flushed and new filter installed so that cannot be it.
13;I have tried driving with the cap off and it made no difference
15;Everything like lines, hoses, holley etc are all brand new.
16;When it starts to splutter I nail the accelerator to open the mechanical secondaries and it takes off again till the fuel runs out in the secondaries then it just dies. I pull to the side of the road and crank it over for about 20 seconds and then it will fire up again.Prior to installing the bypass reg it was real bad to the point where it would only drive for 2 miles and then stall, get it going again and have the same issue mile after mile.
18;Maybe an electric pump will help.
22;The carb is super responsive.Also runs fine around town when warm.....it just has issues when warm and highway driving.

I believe your problem is between the pump and the regulator, and including the pump and regulator.Unless it's sucking air right before the pump.