Pretty accurate if you ask me.
Tolerances in sender rheostat, tolerance in gauge mechanism/needle position can eat up accuracy on that older system. New parts or not. Chinese or NOS. As evidenced by all the parts, combination of parts and adjustments you've experienced. It shows inherent inaccuracy. Don't stress over this small issue.
There is nothing better than brass. Gas won't eat it. Weigh the float with your drug scale. Hold it down in a container of water for a day. Then weigh it again.
It is possible sometimes to move/bend/adjust needle to get more accuracy on the empty end. You only care about the empty reading. You know when it's full when you fill up.
What evidence do you have that the floats are bad? The brass one's are soldered or brazed at the seam. Should not be hard to reflow or seal with modern agents. Lots of carburetor brass floats out there. Floating up and own.
At this stage, I recommend testing the sender and gauge, together, on the bench, to be SURE the accuracy suits you. Very easy to do. Then seal/reseal the float. Then it will all be OK.
