I've done this several times now, its part of my standard prep for X85 Cup Endurance Race Car engine packages.
The standard match is poor between the two cast halfs of the manifold and there is a fair bit of room for improvement. You need to be aware its not just a case of opening it up to the gasket shape as the standard gaskets are massive to (as far as I can tell) aid assembly/remove the risk of port intrusion.
I have a pattern now that I work to and hand machine the manifolds accordingly. There are much better ways to do it than this though, namely CNC and its something that Ben at Angel Works is working on at the moment.
Cheers
M