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Ok so I am new to the forum and never owned a gto but kinda been around the old muscle my whole life and I have a deal pending but there are some things I am treating to figure out with the motor if you guys could please help me out I would really appreciate it!!!! Ill give all the information and hopefully you guys can help me and keep in mind this is what I have been told and not seen for my own eyes so bear with me please, the car is a 1970 gto the engine code is YA=455HO 4speed auto if everything is still correct, the head casting is 6x=400 low hp mid to late 70s the block casting and date are hard to read but i think its 5880557....block date is l070 are these correct? Can any one actually tell me what motor this is and all the info one it I'm jumping around to all these different decoder sites and one number is telling me its one enging then another number is telling me something else like its all mix matched possibly 455ho or maybe 400 lowhp!?!?!?!? Thank you guys for all the help!!!!!
 

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6x heads are often used on earlier 455's when re-built because their volume lowers the compression to 9.5:1 which makes them more pump gas friendly with todays swill.
 

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The block date and the engine code add up, but the casting number isn't right. Should start with either a 9 or a 4.
 

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Mid-late 70's 400's have a 500557 or 568557 casting number. The last number in the date code is the year, "0" would be 1970. Best thing would be to verify the #'s yourself when you see the car. There could be rust, dirt, grease that makes the #'s hard to read.
 
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