The forum at www.clubgti.com
is a mine of information about different engines. Take a look in the technical sections there.
All I recall about the mk3 intake is that it's a bit taller than the mk1/2 and can have clearance issues in a mk1 if used with a mk3 block.
A chap called Jason Moyles has done a lot of flow work/testing on crossflow and counterflow heads - he might be able to offer advice - https://www.jmrclubsport.co.uk