The original engine number for a 1966 engine was stamped on the
block just in front of the right (passenger) Cylinder
head
An example of my 1965 impala ss is F0624HC.
Breaks down as follows.
F= flint manufacting plant
624 = june 24
HC = 327 / 250 HP powerglide trans.
1966 codes
for 327 could be HA ,HB , HC , HF
For 283 could be GA ,GC, GK,GS,GT GW,GX,GL, GZ.
These are taken from my "SS" book so i am not sure if they
changed for the non SS impalas.
There is also a casting number behind the Drivers side head, which you can post, and some one will give you another ID.