Furnace blower sizing question

I have a gimmee used furnace blower.
It has the foillowing dimensions / characteristics:
It has 2 x 12" dia. intakes, a 9"x12" exhaust, 43 vanes, 1/3 hp (1725 rpm) motor with a 3o, hoi 1/2" pulley driving a 9" pulley at the fan.
The motor was made by A.O. Smith and the blower is marked "Viking" with a tough to read catalog number of U 139 and an easier to read model number of 5122534.
I intend to use it for a shop-built air cleaner. It'll blow sandpaper off a shelf from 10" or so ... but I'd like to know the actual or approximate CFM rating so I can have somewhat of a clue in selecting filters for it.
Also, how much clearance do I need to leave in front of the intake ports? I don't want to choke it for air.
I want to size my filtering area sufficient to pass all the air the blower can use plus a little.
Yes, I did a Google search. I came up empty-handed.
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