Several years ago I bought a 1978 Mercury grand marquis from a shady car lot at the time I didnt know this. It was made to look like a racecar with 1970 ford cobra or Thunderbird rally rims and bigger tires on the back. It said 7.5 on the side and I was like that's cool! So I bought it and took her home and it seemed to run OK but it was running a little hot. I didn't drive it much because of that so I thought maybe it was out of time. The engine was chromed out with an edelbrock 750cfm carb and an edelbrock performer series intake so I thought maybe it has a mild RV cam in it or something. When I went to do a tune up I noticed that 460 spark plugs wouldn't fit so I look at the head and saw that it read D0VE-C on it. I read online that they were 429 heads. After over heating several times I parked it because a freeze plug in it started leaking 4 years later with a water hose running into the radiator I got it running again but I need to know more about this set up with the 429 D0VE-C heads and when I timed it with a timing light years ago I remember the engine ran different and it had less take off power and more top end. It does have a 3speed c6trans and 9" rear end if that matters. When it did run I put a clear fuel filter on it near the carb becasue it wouldn't get fuel constantly the filter would fill up then dump fill up then dump. I replaced the mechanical fuel pump still same problem I replaced the water pump and thermostat and it still over heated even took out the thermostat still over heated. No water in the oil or vice versa and I sprayed carb cleaner around the intake manifold to see if there was a leak and couldn't find one. One old timer told me there is a sock in the gas tank that could be degraded and could be getting sucked up into the fuel line going into the tank. Never check that out but would that be related to the over heating? Also someone told me that the head gaskets for the 429 and 460 heads are different and I could have blocked water ports on the engine because of that but I never found that information on the web. Right now I know it has a freeze plug out and an exhaust manifold leak but what explains the initial overheating? Could it have been I was running 87 octane and needed to run the higher octane for those heads? Any knowledge or advice would be very helpful. I'm going to purchase a new gas tank because the one in it leaks. Could that be why the fuel filter would fill and dump? I'm sure I'm missing something but I need advice on fixing this engine. Should I do a new top end gasket set after fixing the freeze plugs? Also should I replace the water pump again and put a new thermostat in it as well. They have been sitting for almost 5 years. While I'm doing that should I check out to see if it has a decent cam? It doesnt lope at all like it has a cam. When I mess with the heads should I remove the cam to see what it is. Any advice on the next step?