This is an old thread but I wanted to document this for anyone else that can't find one of these sending units.
www.collectorsautosupply.com has the sending units in stock for the 36 gallon tanks. They do NOT have them listed on the website. You have to call them directly. Just give them the original part number which is 3895206. Hmmm this is for a 1975 OP was looking for a 1976. Maybe this will still work.
They have been able to track down subsequent interchange part numbers. I picked one up from them for $125. That's significantly less than a lot of places were quoting me to rebuild them.
I should have it in a couple of days and I'll let you know if it works out. They have a return policy so if it doesn't work...no harm no foul. Just my time pulling the tank.