[Matra et Alpine]

As by your own words you've NFI ... then do NOT do anything by yourself unless you have a ready supply of replacement heads or an expert to guide you. Too often the first attempt will end up opening up a water jacket
Especially given inappropriate guidance from "experts" -- come on ht, you should KNOW you cannot arbitrarily suggest that it's OK to flow the valve guide back flush On some engines you'll end up accelerating valve guide wear and introduce horrendous slop !!

Get a tuning book for the head in question. It should give all the suggestions fro doing everything from a Stage 1 - polish and mild port to Stage 4 - reshaping chambers