Will not start as i am not sure if i have the correct firing order for the spark plugs.it tries to turn over, but it appears to backfire,and just runs the battery down

Which engine?

What's the history?... unknown, was running just before tune up and now it doesn't, first start in 15 years,, other?

Backfire would imply timing and or stuck valve(s). The different Ford V8s have different firing orders. Be sure you have identified the specific engine and are referring to the correct diagram.

You can try retarding or advancing the timing a few degrees by turning the distributor.

You may not have properly located cylinder 1 on the distributor cap. You need to find compression stroke of cylinder 1 and bar over engine to TDC. Then remove distributor cap. The distributor rotor will point directly at distributor cap cylinder 1 plug wire tower.

If it's an old engine or has sat for years you will need to do a compression test of all cylinders.

Comment for further assistance.

Hi Timothy:
Old Dude wisdom..........
- Fire at the plugs and
- Fuel in the jugs

If everything else is OK it should run. Not necessarily well, but it should run.

What to do?
1) Check for hot blue spark at the plug electrodes
2) Check for fuel in the carb.

If you have spark, give it a shot of gas into the carb, and give it a try. If it doesn't start, backfires, or catches fire, you'll need to double check the timing and plug lead firing order.

Not kidding! Don't set fire to it!.

first up check that you have put the spark plug leads on the correct plugs in the firing order as back firing comes from having 2 leads crossed. Have a pressure test done on the coolant system to find the leak . If it just started after the back firing I suspect that the inlet manifold may be corroded at the water cross over section and the back firing may have loosened the gasket enough for the leak to mstart

This may be due to a brunt valve or misadjusted valve, the back firing is due to the intake valve being open while its firing in that cylinder.
Make sure all wires to cylinders are correct and not crossed.

My first thing i would look at is the firing order, may be a crossed wire, even if it starts and runs you may still have a crossed wire, then if firing order is correct then i would move to checking the adustment on the valves for that cylinder,
Make sure rocker arms arm tightened correctly and push rods are in correct locations. i know that some engines like the 3.1 have diffrent size push rods for the exhaust and intake, remove the valve cover and see if that valve is closed, or try a compression test on that cylinder and if it is low then you have a bad valve or it is not adjusted correctly and last the timing chain is not aligned correctly.

0.042-0.046 is the correct gap for your spark plugs,If it is backfiring now and was not before make sure you were not sold platinum bosch plugs and if you are using factory plugs,then I be looking for a faulty spark plug.Also make sure the firing order is correct.

Still have a wire crossed somewhere. That is why you are getting the backfiring. Recheck firing order and make sure it is correct.

Do you have the "firing order" correct? In other words, is each spark plug wire going to the same cylinder it was on before you changed the plugs? If you have done nothing else but changed the plugs then this is the most likely problem.
If when you try to start the car it backfires through the exhaust or backfires through the intake. Then this " IS" the problem. Good luck.

1-4
5-6
2-3
double check first

Make sure the part store gave you the correct plugs and wires. Consult a third source for a firing order.

 Check compression possible tight valve lash, if ok Check for vacume leak, make sure firing order is correct, check power to injectors an make sure injectors are working.