OK so I had my 289 machined and bored .040+ and crank ground -.010 and I rebuilt the engine myself w/ rebuilt GT40 heads. I used the correct bearing, etc. I noticed a knocking after the rebuild, which sounded a lot like a lifter knock, and I went all over the engine w/ a stethoscope and was not able to pin down the knock. I started reading and thought it was rod knock so I pulled the pan and all the rods are nice and tight but I noticed the oil was a little milky. So, I started thinking maybe I have a blown head gasket? Maybe the GT40 head valves and my flat top pistons are hitting. Maybe I put a piston in backwards or maybe it’s just a bad lifter(s)? I pulled the intake and then the drivers side head (still need to pull passenger side head) and I found no evidence of the valves hitting the pistons or a backwards piston. I did discovered that the back cylinder was much darker than the other cylinders and the back plug was much darker. I then noticed the head gasket I used doesn't have holes for the back coolant passages (see pics)? Also in the photos all the cylinders look clean as I had to wipe down the cylinders because when I pulled the heads they filled w/ coolant, so I drained them, wiped dry and soaked w/ WD40. So in my amateur opinion......I used the wrong head gasket which caused the back cylinder (s) to get too hot, blow the gasket(s) and start allowing a small amount of coolant in the oil which screwed up my lifters and created the knock. What do you think? My plan of attack is to replace the lifters and reassemble w/ the correct head gaskets, add fresh oil and see what happens.... Thanks in advance for your help.