Since Paul Yaw claims intake runner air velocity is everything
polish your manifold and intake runners with this tool. Works
great chucked in a die grinder.

It is called a "Cartridge Roll". Made by 3M You will go through
a BUNCH. Comes in various grits. Rutland Tool. 800 289 4787
B&R Supply charges less. 805 656 0715

Or you could send it out and have it extrude honed after it was welded
up.  http://www.extrudehone.com/

Paul Lamar

Here is another opinion on Extrude Hone.
http://www.shotimes.com/SHO4extrudehone.html

Paul Lamar
 
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