68 gto came with 3 diff heads, need help

i just got a 68 gto, the guy i bought it from said that 2 of the heads were off of a 350 and 1 was one of the origional ones from the 400, but it looks like every one is different, 2 of the heads are the same valve size and the other one has a biger exhast valve, i need to find out what the head with the biger exhast valve came off of, here are the casting #s of the 3 cyl heads: on center exhast port the first head reads 4X and on the other port it reads gm7, the next head on the center exhast port it says 6S on the other port it reads gm4 and the other port it has an X, the next head it reads on the center port what looks like to be a 16 and on the side ports it has just gm and on the other port a 3, could any one help me or point me in the right direction, i dont know if this helps any but the motor is stamped YC does any of those heads match the motor?

Chad, the "4x" head came off of a 1973 or 1974 motor... the "6s" head came off of a 1975 or a 1976 motor and the "16" came off of a 1968-69 or 1970. The "16" head should have a "number/letter" code on it. (example: B098) which translate to February the 9th 1968... the other heads should have this "number/letter" code as well. Your "YC" motor should have these same "number/letter" code on it as well. Also it will have a number on the back top of the block ( left- rear as your looking at the front of the motor) which will tell you what displacement the motor is. (example: 9790071 which is a 1968 400 CID block). The "16" head will have the larger valve and it is the one that probably came with your motor. Hope this helps Chad.....Joe

thanks