Pontiac block casting numbers???

I am doing a little research on an engine that I plan to put in my 54 Catalina. I have searched and am having trouble figuring out if its a 350 or a 400.The casting number on the block behind the #8 cylinder is 500516. The date code is 77. Does anybody have any info they could share???

 

Arent thay just cast in the side of the block what size it is ?

 

Its on the right side just above the engin mount

 

I checked there and found nothing . Someone was nice enough to grind them off. I have found other casting numbers on the heads, intake, timing cover and carb.
Front of p***. Cyl. Head - XC
Intake -D187 525355
Timing cover - 27P369409
Carb -17057262 1337ARK

 

I have been able to decipher everything except for the block casting number. The heads, intake and carb all interchange between a 350 and 400.

 

Are the letter codes on the front of the block on the right side just under the head serface?

 

nope nothing there. I kinda found that odd

 

Maybe a replacment block my 400 is a 77 and the casting # is 500557

 

http://www.pontiacpower.net/intake01.html

 

try www.wallaceracing.com

lot's of good info there.
I'll also check my book later, but I'm at work now

 

http://www.antiquecarparts.net/tech02.htm
http://www.pontiacpower.net/engcode01.html