Just FYI...
An SEFI equipped 5.0 will run with the std firing order. I've done it myself. Only 4 of the cylinders fire differently and the ECM does attempt to compensate for that. It dooesn't run good, but it will run.
Well, I put it on the HO firing order and it wouldnt start. We fooled with it for about two hours trying to figure out what the hell the problem was and ended up trying to put the non HO firing order on and it kicked over. It ran a little dumpy, but it was oddly smooth.. So I figured the problems were related to the fact the timing is just kinda.. bleh at the moment. But while out in my garage tonight I started listening to the engine and noticed the wires didnt look right to me (didnt look right speaking from the non HO pattern) so I removed em all and for some reason just decided to put em on in the HO pattern for the hell of it.
So essentially it wouldnt run with the HO order, but would run with the non HO order screwed up slightly, then would run with HO order.
I have no idea how I pulled that one off. I am either the greatest mechanic ever, or the worst.. and I vote the latter.
So....it's now running in HO firing order....correct. This should alleviate your feelings a little. At the very least, you know now that it's a roller block (or roller block conversion) with an HO camshaft. And from what you know, you’ve already got hypereutectic pistons in there, which isn’t such a bad thing and the stock HO intake. Now all you have to wonder about is the heads and you’re all set. If memory serves, when you pull a valve cover off, the E7TE heads have the letter “T” stamped on them near one of the pedestals of the valve train. This signifies “Truck” which is what these heads were pirated from, before they made their way to the Mustang.
On last thing....make sure to separate the number 7 and 8 sparkplug wire from one and other (just swap them to opposite sides of the spark plug wire looms). Having them directly beside one and other sometimes causes misfires that could lead to big problems at high RPM.