Well, just to clarify a little further what Fraser said, the physical dimensions of the MAF are not the issue. The original "MAF mod" that was outlined on MN12 Performance borderlines on stupid in the first place.
1) The stock electronics on the 89 MAF limit readable airflow to about 450cfm. After 450 cfm, regardless of how much air may or may not flow through the meter, no more air will be recorded, which means that the electronics will limit fuel delivery to the motor before the CFM limit of the MAF is reached.
2) The stock electronics on the 95 MAF limit readable airflow to about 450cfm. So even though the 95 MAF flows 680cfm, the electronics quit the program at the same basic airflow as the early MAF making the bigger MAF pretty much useless.
3) The MAF Mod detailed on MN12 Performance attempted to increase the CFM capability of the MAF without changing the sensor output. This is the reason for drilling out the sample tube. This is fallacy. Since the sensor output is still limited to 450cfm, there is no possible gain from this mod. All you can possibly accomplish is to create more opportunity (through more unmeasurable air flow) to cause severe top end engine lean out.
But even that really isn't possible because the stock injectors will also max out long before the newly "ported" MAF runs out of CFM capability. The MAF mod idea was an unfortunate combination of "carb technology" thinking and EFI that simply didn't work.
4) Now if you really wanted to make some progress, what you'd do is port out the MAF (and remove the center post of course) but NOT alter the sample tube at all. If you do this correctly, you can increase CFM to about 750, and change the calibration from stock (30/36lb injectors) to a 60lb calibration. But, it's not a precise science, and depending on how you do the porting you could end up with anywhere from 42lb up to 60lb calibration , so having a QH or similar tuning capability is absolutely necessary to pull this off successfully and without risking damage to your motor.
So in the end, the stock 89 MAF is perfectly fine with the stock 30lb injectors. The 95 MAF housing will work ok on an 89 with stock injectors also and may result in a gain of about 3rwhp at high rpm, but neither MAF is any good with bigger than stock injectors without modifications (NOT the "MAF Mod" referred on MN12 Performance) which WILL require tuning.
Therefore the average SC'er is going to be way further ahead by buying upgraded injectors and MAF's at the same time and finally by buying a chip that is calibrated to recognize both items.
Clear as mud?