I was re-reading your second post and am guessing that you found a block casting number of 3884257.

I looked a little and wonder if what you think are 8's could be 9's.

Block casting # 3994257 shows up as a 1967-72 250 with a head casting number of 3927763 or 3895052 according to this site.
(Look almost half way down the list)

http://www.butlers-a1.com/chevym.htm

Just something to think about.

"Nothing astonishes men so much as common sense and plain dealing." - Ralph Waldo Emerson