The difference in the senders design or shape is in year models. If the OEM sender is 1973 or later It probably will be identical to aftermarket. Well almost. The very cheapest aftermarket sender has a plastic float.
Problem I saw was the new float arm was way shorter. That changes the ratios. That makes it so at first the gauge reads Full to 3/4 full for quite a while. Then goes from 3/4 to 1/4 full fairly steady. Finally it goes from 1/4 to E VERY fast.
Now with the right circuitry that situation can be overcome. But they don't. You can see this float/rheostat assy is used is lot of different Repro Sending Units. Dodge, Ford, Chevy. One size fits all. :violent1:
Here's a comparison picture. This is a side view of the pickup as they would be installed in a gas tank.
