I've always felt hat the reasons to go metric are cost and reliability--and that running a velocity stack would fu'up both of those. Much more frequent oil changes, difficulty to keep it from going way lean & running hot.
There are both DIY and store-bought solutions to making he air-box go away.
Search on "Grampizing' @ Gadget's fix-its, VROC.og etc. for home-brew DIY
for a store-bought fix, including jet kit & a new A.C.
I think that unfortunately most of Thundermfg. A.C.'s are big honking units :-(