Well BC,
It doesn't appear we have exact documentation as to the size and given the fact the PO replaced the tank (exact replica or not) you may not know for sure until you actually open the hatch and take a look.
That stated, mechanical gauges especially the floating arm type are not known for accuracy. The behavior you are observing is typical. The reed type (sliding ring) is considered to have a higher accuracy but I believe this is because it doesn't "bounce" as easily as the float/arm type (though the reed does have a float). Since you just changed the sender out, you probably don't wish to buy yet anther gauge for a slight increase in accuracy. Still, the reed/sliding ring may help with readings on an analog gauge. I have mine on an LCD reading gauge and it's far from perfect.
Lastly,
Since you have access to the sender and therefore the tank and you seem willing to remove the sender to try adjustment, you could try this:
Remove the sender.
Get a wood dowel long enough to touch the bottom of the tank and while you have it in place, mark the dowel where it exits the tank. Now mark the dowel halfway between the bottom and top mark and that will represent about half a tank of fuel. Rewire the sender but leave it out and place it next to the dowel. Line up the top mark and the bottom of the sender flange (where it mounts to the tank)and observe the travel of the float arm and its relation to the dowel. With the sender wired, you can power up the fuel gauge and watch the needle travel. Now you can get a good feel for what the gauge is reading compared to the depth of the tank. You can also measure/mark different fractions of the tank depth on the dowel and again see the relationship between the two.
Pinpoint accuracy? Not hardly but at least a better idea of your remaining fuel reserves than what you have now. I don't know what type float mechanism you have but I doubt you have a lot of adjustment available to use. Still, basing the relationship between the gauge (needle) movement and the dowel and using 27 gallons (smaller of the two tank sizes) as a basis, you can better judge what you have left.
I don't really know if this is worth your time but just my thoughts. I did the same scenario with a reed type on my LCD gauge and created a little spreadsheet. Still, I rely on a Floscan metering system for fuel/performance management but the exercise with the reed gauge was useful too.
Good luck.
