ECM troubleshooting

[mudchucker]

Ok guys, here is a good one for you. Hopefully someone can point me in the right direction for some help.
I went out to start the 5.0 and nothing. It was turning over, but wouldn't fire. I checked all the normal stuff and there was no spark. So figuring it was the module on the side fo the distributor i replaced that and still nothing. So i wondered if it could be my ECM and i have a couple spares lying around from other cars so i put one in and it fired. The problem is that i have tried both of these other ECM's in this engine and they never run correctly. Here is my question, i have all the part #'s from the ecm's, how do i know which ones should work for my application? sorry for the loing intro. If you guys need the #'s off of the ECM's let me know and i can put them on here.

Thanks again.