You're right, there's nothing better than doing your AFR tuning on the engine in question.
However, the difference between what you are running now (a calibration designed for a 5.0L Mustang with a 137 firing sequence, 301.08 cubic inch displacement, 19lb/hr injectors, smog legality, etc...etc...) and the *right* calibration for the parts that the EFI system is actually connected to is going to be night and day....even before performing the fuel trim operation on the engine in question. You can do this baseline tuning stuff yourself with the tweecer, then you'd have the ability to take it further if you so chose. If you have massflow do the baseline stuff, that's probably where it ends because you don't have their tuning software/system. The only thing stopping you from doing the baseline calibration stuff yourself is knowledge of the massflow curve in Volts vs KG/Hr. If they'll give you that, you're home free and you could have this put together in an afternoon.
BTW, I ran a google image search and ran into this...which looks familiar I hope. There's an applications list on the page that suggests this is a GM type maf...so whatever you have is probably a GM type maf. Probably not this one though; I'd suspect they'd use something from a performance vehicle so it has enough measurement range (Corvette, Camaro). Again, just a guess.
http://www.cheaptruckparts.com/produ...ucts_id/27187/
http://www.corvetteguys.com/mass-air-flow-sensors.html