the Keihins are very good,
BUT way too big unless you have done serious reworking to the cylinder head:
and done the usual: Megacycle+Kibblewhite or RD, Carillo, JE piston+rebore, much larger headers etc. etc. etc.
The carbs are expensive but nowhere near what the tuning itself will cost.
If you are running the stock engine+exhaust system, anything bigger 34 or 35mm ist simply too large for the manifold vacuum. The beast will not be drivable.
Use the search engine and read what Wonky has written, and what I have written as well.