I've been running Fitech for 2 years now on my 406. Had a Mighty Demon 750 on it previously for 10+ years. It was a good carb. But, it is so nice to start this truck and drive it like a modern vehicle. I hate calling customer support, so I did my homework. I honestly can't even guess how many hours I read at night about self learning efi. Read all you can, be patient, make one change at a time, and you'll have you truck running like a song.
Most, not all or course, problems with the newer TBI systems is that they are not always installed properly, or some little thing that gets overlooked causes nagging issues. I've had 4 square bodies with TBI, and not one of them drives with the responsiveness that this Fitech system does. And now that I've spent over 2 years with this, I make little adjustments when I see fit, and the chances are immediate. No burning chips or crap like that. Just my $.02