[ByronRACE]

The very first thing I'd do is get a wideband oxygen sensor on board and weld bungs into both of your exhaust pipes near the collectors so you can install this sensor and monitor the air/fuel ratio. Without that, you're simply guessing. I can't tell rich from lean in an EFI system because it's dynamic. It might be rich one moment, then due to the inputs...swing off lean. Or, it may fluctuate wildly from one range to the other. It's not like a carb.

Additionally, white plugs and white oxygen sensors aren't unusual because the mixture runs 15.3-14.1 as a matter of normal operation.

I'm not in agreement where MassFlow puts the maf. MAFs operate on the principle of laminar flow. You can't have laminar flow with throttle blades an inch or two away, so I would expect your AFR error to vary with throttle angle as it disrupts laminar flow. A lot of bronco conversion customers try to clamp the MAF on the end of the throttle body in 5.0L conversions and it results in horrible drive behavior that is very difficult to mask with calibration fiddling. I don't know how they get away with that with most cars, and can tell you that I've seen from data logs, it doesn't work as well as the factory maf configuration. I don't know if your problem is maf related, but it sure sounds like it could be.

Once you get a wideband on there, and as long as you can run the engine at WOT, I'd put it on a dyno or go find a long stretch of road and see what the WOT AFR is above 3000rpm. It should be more or less steady. Once you know what that number is, you know how far off your open loop fuel is from where it should be, and massflow can make an adjustment to the entire calibration by that offset amount. That alone may fix your problem.

Without a wideband, you have zero data by which to judge the next move.

The factory 5.0L Mustang box is calibrated at about 12.3:1 AFR at WOT. I don't know where these guys calibrate the box, or whether it's reflashed or what have you. The box I saw looked and behaved like a factory 5.0L Mustang MAF SEFI box, 5spd family, and the AF in there behaved more or less like a factory MAF at WOT. I was able to calibrate it by treating is just like a Mustang. The box itself had no markings; the Ford label was removed and the entire box was spraypainted black.