Finally got to replacing the eater core today. Indeed it was quite easy, about one hour work! So thanx for all the advice, glad I didn't take the fender off... Simply release the bolts from the engine side, then pull the entire system off on the dash side, and change the core...
Core (ordered at AMES as 67 firebird heater radiator) was not an EXACT match (slightly thinner and less wide), but the tubes were okay, so it fitted without problems.
Some pics for people that are changing theirs in the future:
