Header tank replacement issues

Hello, I recently had my header tank replaced with a Univair tank that I removed from a salvage coupe of same type (in fact it had been built within couple weeks of mine in 1946). On my tank that was replaced, the fuel lines exited the bottom of tank and on the new the only line exiting the bottom is the one that feeds carb, the other two line for header tank pump in and overflow are on back of new tank. I was able to retrieve the necessary lines off the salvage aircraft to adapt to the new fuel locations. This in itself is pretty straight forward.

So we ran into an issue when I went to pick up the coupe, simply put, the fuel does not seem to be draining on the overflow line into the left fuel tank fast enough to drain off what is coming into the tank and over short period of time (thankfully during run up before take off) the header tank overflowed and ran out of the header tank cap vent,etc. After idle down, the fuel level will slowly drain back down to the normal header tank fuel height. This was repeated a number of times.

Set the clock now two weeks later and still trying to figure out why the new tank does not seem to drain overflow fuel as quickly as the old tank. I suspected a fuel line issue on the line that goes from overflow port down to the left fuel tank as there is an adapter to convert from a larger line down to the original smaller fuel line going to the tank. Upon checking that the mechanic found no issues or contamination.

I also suspected possibly a venting issue on the left fuel tank but both the left and right fuel tank seem to be venting properly.

This is a simple system, it feels that if the earlier tank was working fine than this new one should as well. I have some theories still on venting of the system but curious what others may have for any ideas at all.

Maybe I should (if this is even an option) replace the entire overflow line with the larger size from the header tank down to the left fuel tank rather than have it adapted/down sized to original mid point in the line. With that said, it is designed to have this adapter in place so I feel that may just be a fix for a completely different issue that is yet to be found.

Any ideas here would be appreciated.
if easier to talk through on phone rather than in text please feel free to call my number at 608-445-8546. I am in Wisconsin.

thanks ,
Keith