There’s very little difference between the 2 and 3 pipe. If you run a shorter flex, you will get the same result. If you read the IKF rule book, it gives you all of the dimensions. The only difference is the volute. Use a 10-10.5″ flex and get the same result as a 3 pipe with a longer flex.
Check the rules section of Tri-C Karters or F100 karters for the direct flex length comparison.