If it's Smaller than boreing it to fit is the best way for making power increases. Even if it takes some time. A spacer that is one size fits all Is just snake oil.
Edges & lips Kill cfm flow and The whole point of add HP increaseing Mods is to Make it the BEST YOU CAN.
A sanding drum on a die grinder and Some music,a few cold ones
, make for a good weekend night out in the shop.