I looked into raising the domes on a set of small valve heads. It ended up cheaper tracking down a set of real 850 lemans heads. ( I have a 77-78 850 LeMans Engine that I plan on building. it only had one remaining small valve head on it and the other piston shows signs of eating a valve. )
The difference between the small and medium valve heads are pretty minor and are not immediately apparent to the naked eye. If the heads are off, I highly recommend some port work. I have an 850T that is mildly hopped up. The work on my T heads was minimal and made the bike pretty happy.
Maybe you could get away with port work, and thicker head and base gaskets. We used thin copper head and base gaskets on my 850 T when we set the squish to .040 my butterfly robert frost