Hmmm... 66 heads on a 76 block. Maybe things are different in Sweden but in the states 66 318's were Poly engines and the heads are totally different than 68 and up 318 LA heads. Also not knowing the cam makes it pretty much impossible to guess the hp as there is a big difference from the 68 340 4 spd. cam and the 73 automatic cam. And there's a few in between. Sorry but we'd need more info. Do you have the head casting numbers?
BTW: A 600 carb is just fine for a 318 but that's as big as I'd go unless it was a full race engine.
And you did great with the spelling and welcome to the board.
Addendum: I just looked at your web page and you definitely have 68 (or newer) la style heads. Great looking ride!