I have a blower I removed, from the old air handler when my AC was replaced.

I would like to use it as a fan in the garage. It moves a lot of air. I figure the duct work would restrict flow and therefore the blower was designed for some restriction. Plus there is what looks like a piece of metal on the outlet side to that blocks part of air flow, or that's my best guess.

Do I need to restrict the air to the fan? If yes, can I set it by using an amp meter?

No need to restrict, depending on what you have it may have multiple speed taps.

Run it, put an amp clamp on it and compare the reading to the FLA rating on the motor name plate.  The one I'm currently running in my shop, I built a reducer for the output that looks just like a commercial floor drying fan.  It had a slightly high amp draw before I reduced the output but runs perfect now.