Can anyone help me find out about the engine I have in my 68' Firebird? I know it isn't the origional engine. The front of the engine has YM stamped on it and back around the distributor I found a 73 and DN. The heads have 46 stamped in the middle.

Hi
Hope this helps.Well here goes ,there is a 68 350 ym block,also 74,75 400 block.The 46 heads could be 69 but probly 73-74 350 d-port.I'm new at firebirds so i could be wrong.But i'm pretty sure.
Have fun
Bmxer