Will this fit a Goodman AMST42CU1300 air handler? If so, can a 25" x 10" return be cut into the side of it?

Yes, the FB2128 should fit the Goodman AMST42CU1300, but I would not recommend cutting a 25" × 10" return opening directly into the side of the air handler unless Goodman specifically approves that modification. The AMST42CU1300 has a 21" × 21" cabinet, and its approved return-air configurations are designed around the factory-provided openings and duct connections

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