Ok so I just put on a 48IDA carb and a custom built intake manifold along with a holly red pump with a holly fuel pressure regulator (1 - 4 1/2 fuel pressure)...... the problem is that I lose fuel pressure around 5K rpm.

 

Sounds like the pump cant keep up. Seems odd as I assume a holly pump should be more than sufficient. I used to run a weber DCO sidedraft, and although its not an IDA, I remember it needed around 6psi of pressure for it to be happy.

You might try reposting your question in the 1st gen section as I am sure there are people with a similar fuel pump / regulator / carb setup in there.

 

How new is the pump? I had fuel pressure issues with my Holley blue not holding pressure at 6 PSI consistently after installing it from new. Here's what I recently found after it finally just quit working: https://www.rx7club.com/showpost.php...&postcount=110

It has since been replaced by a Carter a couple days ago

~T.J.