The MAF meter for 94-95 GT and Cobras are calibrated for 19 ln injectors. As long as this one has that stock calibration it should be calibrated properly for a '91. You might run into a problem with the wiring harness - I have no idea if they are the same, but you could probably tell by looking at them. I've never really heard of a MAF being too big. I understand you can hurt low-end power with a throttle body that is too big, but I don't think there is a down-side to a large MAF.
A forum community dedicated to Ford Mustang owners and enthusiasts. Come join the discussion about performance, production numbers, VIN decoders, modifications, troubleshooting, power adders, DIY tutorials, and more!