I am putting in a racing fuel cell in my 84 with the 2.5 TBI. It will be using a holly fuel pump and regulator. It is my understanding that if I am using the holly regulator that I can terminate the return fuel line as I will not be using the factory regulator.

I am putting in a 350 in a year or so and I would hate to run a line and remove it later.

One of my concerns is the quality of the regulator and what it could do to the throttle body if it fails. In this case would it be best to use both in case of failure.

You're going to have to run a new fuel line anyway.
What's the problem in running a return line next to it as well?

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The carborated 350 going in does not need a return line. I was hoping to prevent running the line and removing it later.