what type of motor do I have

[chadistre]

Hey guys this is my first time on a forum. I recently purchased a 1972 ventura. The motor was rebuilt and never ran. Now it has been sitting for ten years. I am in the process of tearing it down to change all gaskets and put new break in oil in it. I am curious as to exactly what motor I have here. The block has 400 on it which I take to mean cubic inches. It is also stamped GM-5DN. I am curious to know what that means. On the passenger side top of the water pump there is an XA stamp. On the passenger side bottom it is stamped 271N32598. I take that as the serial number. Am I correct. Above the center exhaust port on the heads there is a 6X stamp. To the right between the center exhaust port and the right one it is stamped J156. Can anyone tell me what all this means or tell me where to find out. Since I strarted tearing the motor down I have learned that the motor is bored 040 over. it has h-beam pistons rods. It has hooker headers on it and an edelbrock intake. The numbers on the motor is my concern right now. any help with that would be greatly appreciated.