It depends on how crude you want to be
Crude.... Just use a large flatwasher or lockwasher to make up the difference.
Also crude... you could make a ring out of 1/8th inch dia rod...(this is what I would do..weld or braze it up))
Less crude... Make a short cone section out of sheet metal... (not easy to do correctly)
The overall length of the pipe is what determines what RPM you make your peak HP at.. the longer the pipe the lower the RPM your peak HP occurs at.