First thing I would do is pull the valve cover and take a look see. Maybe the rocker shaft loosened up, or maybe the rockers are worn. Do you know any history about the engine?
History of the engine is - it used to frequent the local drag strip before being parks for many years. all I know about it is that it's a 318 block with X-heads and 340 intake, and some aftermarket cam. everything else is unknown. Could very well be stroked, certainly has some work done in order to run the 318 with the X-heads... I did check the rods to make sure they were straight, which involved removing and re-installing the rocker shaft so it's not that...
I need to do a comp test, I believe when I did one many many years ago that I had 1 cylinder with low compression, but I'de need to re-test to be sure