First of all,thanks for the answers !
Alright, I removed the valve cover and checked the rockers, they were all rather tight except for one which I could slightly click from left to right, but none could be taken from the pushrod or get compressed by hand..the loosest one isn't even near the source of the sound, since it's more present at the second cylinder of that row and the loosest rocker was the last of that row...
Is there any other way to check the lifters? or should I turn the engine by hand and check the rockers with the crank/camshaft at different positions?

The engine does in fact run rough when the noise occurs , you can clearly hear it miss when listening to the exhaust... hard to describe, it almost sounds like there's a racing cam in there from time to time...

@"Matthyj"
what exactlxy do you mean by "smogger heads"? sorry if that's a stupid question, but i'm new to those engines so please forgive me
The distributor and carb look all stock and rancid:





And I also found something else:
All the "emission"-stuff got deleted by the pre owner, the metal lines on the header are cut of and crimped together on one side, on the other side the big line which feeds the smaller lines going to the cylinders got plugged with...well, I can't exactly tell whats in there...however, the engine light also got blacked out with a marker, but you can still see it lighting up in the corners..




Thanks