Mustang engine I.D. Help

[hotrod5125]

I have been trying to retime my ford 302 Mustang-motor in my 51 F-1 with no luck .The person i bought the truck from said the engine came out of a 1978 mustang.A guy at my local speed shop told me i might have a different engine than i think and the firing order is diferent on some of the small block fords??? The engine ID number is D8V8-6015-A3AZ.I have the firing order on the distributor cap clockwise rotation 1-8-4-5-6-2-7-3. With the rotor button piont at #1 on "TDC"piston (pass side forward most cylinder).I have the pistons in this order pass side front as 1 then 2 -3 -4. Driver side front as 5 then 6 -7-8 ??? I'am a welder/fabricator not a engine guy and i think i'am proving it trying to make this truck run with this motor out of a Mustang. Thanks for any help please.
Steve