A standard 69 Caddy had a 26 gallon tank which I think carried well into the 70's. The hearse left the factory as a 'commercial chassis' and it was up to the 'coach builder' how the car was finished. It would depend on who and where the car was built for. Often times a hearse never made long trips so they may have used a fairly small tank. I know of a 72 miller that has a 50 some gallon tank. The car was originally a hearse / ambulance combo in a smaller town that often had to run a 300+ mile trip to a big city so they must have ordered it with a large tank.