Can you help me identify my motor?

[parobellum]

New to me 1969 C10 - originally a 396 w/AC - currently has a big block and vintage air system and runs great - I am very happy. Motor ID has been odd as I was told it is a non-original motor.

I managed to find the intake manifold number (see below) 3931067 which i decoded as

3931067 1969 396 325 L-35

I also found the date code E6|9 I believe (pic below) so May 1969?

I finally found the block casting # after an embarrassingly long time and even buying an endoscope (mostly because it looked cool. And it is.)

To me it looks like 3855961 P which decodes as 1966-67 396 325, 360, 2 bolt

I am assuming based on the manifold its an L35 (396/325).

While I think I am correctly assuming it was a swap, I cannot find the vin number despite watching youtube videos, looking at every pic on the web . Can anyone highlight my stupidity and help me out?