hearse three gallon blues

[fearshirefarms]

I have a 1985 Cadillac Fleetwood Bram hearse. The gas tank is full but it only runs on the last three gallons of gas. The intake fueling spout was moved down to the rear license plate when the hearse package was put on. I don’t know if this is the air ride gone bad, a plugged exhaust vent or some other problem but it keeps my hearse close to home. I have dropped the tank myself three times and haven’t really gotten anywhere. When I tell this gloomy story to mechanics no one wants to touch it. Haunt season is coming and it is time for the beast to prowl. Any ideas how to get-r done?

[Dickey]

I realize you've already been down this road to some extent; but what does the gas tank weigh after you fill it up?

Whether or not your tank is actually full after filling up at the pump would tell you if your issue is a fuel pickup detail or if it is a fuel filler detail (more likely since that was changed). If it is heavy (1gallon ~ 6lbs), then you have some issue with your fuel pickup (venting issue or otherwise). If it weighs less than 25lbs after filling up, then I'd venture your fuel filler setup is not adequate.