I would get a aftermarket temp gauge so you know where you are really at. You can buy a mechanical one that installs in the upper radiator hose pretty easily.
Depending on how bad you want the oem one, this can be a good solution.
Some folks don’t like the radiator hose senders because they...
M&H from year one are copies of the originals. You don’t need a lot of electronic skill to replace the originals, since they are the same. This is what I did. The only bad news is all the accessories harnesses (like AC) are sold separately, so it drives the cost up. Use dielectric grease on all...
They are not that hard to find if you have about $600-900 in your wallet. They even make them new now. If you don’t want to the spend that much, the 72-74 a body 340 360 manifolds are a lot cheaper. That is what I have on my 69 Cuda.
I chased electrical gremlins for a long time on mine. If I had it to do over again I would buy all new M&H harnesses from year one and change it all out.
Do the taillights work if you swap the LED bulbs back in? Looking for clues…..modified wiring is hard to troubleshoot.