1987 ford E350 460 7.5L backfires

Engines that sit for a long time suffer from the fuel going bad. There will be so much gunk in the tank, removing it and having a radiator shop boil it out is the best course of action. Backfiring on engines that have been stored is most often from sticking intake valves. The bad fuel leaves deposits on the valve stems causing them to be slow (or non) closing. A sticking intake valve will lean out the air/fuel mixture to the point the engine cannot start without the starting fluid. Is the backfiring random or does it seem like it is occuring on a regular basis? If it is one sticking valve, you can determine which one by pulling plug wires one at a time until the backfire stops (assuming you can get it to run long enough to tell). With fresh gas and Techron added to the tank, it often goes away. If not, worst case you'll need to pull the head(s) to fix it. You might want to pull the valve covers and watch the valves as the engine is cranked over. A sticking valve is pretty obvious. I've had luck freeing stuck valves using B-laster penetrating oil without pulling the head. I'd remove the valve cover and since the valve is probably slightly open, a fitting to allow you to apply compressor air pressure to the cylinder won't really do the job holding the valve closed, I insert clothesline into the sparkplug hole until I can't get any more in then turn the engine by hand to jam the clothesline up against the valves. Now the valve cannot fall into the cylinder when you remove the keeper, spring cap, spring, and valve stem seal. Now you can squirt penetrating oil down the valve stem. You can wrap some electrical tape on the valve stem and after rotating the engine a little to get some free play on the valve, use a pliers to rotate and move the valve up and down to help the penetrating oil do it's job. Beats pulling the head. I'd also suspect something in the EFI system is plugged up if starting fluid is the only way you can get it to run. Since it will fire on the starting fluid, the ignition system is probably OK. You need to put a gauge on the fuel rail to make sure the injectors are getting correct fuel pressure. A "noid" light that fits your Ford plugs into the fuel injector harness and will tell if the injector is getting voltage to open (flashes each time the injector is pulsed). If that is happening and the engine is not trying to start, the injectors themselves are probably plugged with stale gas deposits. Would need a new/rebuilt set to fix that. Not much way to clean them properly in the field. As you can see, letting gas go stale can cause a myriad of problems that can get expensive to fix. Old time mechanics would often dribble automatic transmission fluid down the intake to help lubricate a sticky valve (and fog the neighborhood for mosquitoes). On modern engines this will wreck sensors. Don't do it.