I'm assuming you're planning on running the 2.5" pipe all the way up to the sensor and then use a coupler?
If you're going to be tuning the fueling (which you really should, even if you're just using the unit for a MAF translator) it'll be fine, if not, you'll need to determine how far off the measured airflow is from actual as you're not going to be flowing as much air through the outer areas as the inner... I had this problem.
You may run 3.5" pipe before the sensor for a foot or so, that'll allow the airflow to even out well prior to going through the MAF's screen.
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