I posted this in the Carb LS forum, but figured I might get more response over here, sorry for the repost but I didn't know how to move it.

Guys, I need a little help. I have a racetronix intank pump with the stock F-body bucket installed in my tank. I have a aeromotive 13301 regulator plumbed inline. I have the return going right back in the tank with no more than a #6 AN elbow. both the feed and return are #6. I have the regulator mounted to the tank, but it's upside down. Here's the issue, even with the lightweight spring installed and the adjustment screw backed all the way out, I still have way to much pressure at the carb inlet (20+psi). Any ideas on what may be the problem? Here's a few pictures that might help explain.

 

The install instructions say that the low psi spring is in the reg when it's shipped.
Are you sure the spring is the correct one?
Sounds like a couple things could be going on:
A. Both the supplied springs are the hi psi pieces.
B. If they are marked, they are marked incorrectly.
C. You have them backwards.

Since the psi you are getting, [20], is dead on what the regulator min/max is for the hi psi spring, I'd look there first....

 

Also in a carbed application the return line needs to be rather large. Typically a -8 to minimize flow restriction.
You may want to disconnect the reg from the sender to check flow restriction.
Depending on the sender mods, the return passages inside the sender may too small.
You may have to run a line directly into the tank instead of through the sender.