512Stroker said

You said that you have added a 1/2 spacer.
Is that a black composite thermal spacer between the carb and the intake manifold? Or something different?
You will need to put a fuel pressure gauge in line between the pump and the carb for trouble shooting. Taking hot and cold reading will give you some indication of what going on, especially when it locks up.
Fuel pump may be weak.
Try to avoid the little cheesy fuel pressure gauge that thread mounts inline, they are very inaccurate. Locate one that is on a whip hose.

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