As quoted by Spinner on Google:
Yes the Block Code on the front right of the block (passenger side Front) 2 Letters for most 389 and 421's
On the block just below the right head ...
If you know what year I have 85% of each list...
Example
65 389 256hp Manual trans WA WB
65 389 256hp Auto trans ... YA YB
65 389 333hp Manual Trans WE
65 389 325hp Auto Trans ... YE YF
65 389 RA 360hp Manual WS ... Auto YR
65 421 338hp Manual WG ... Auto YH
65 421 376hp 3X2 manual WJ ...auto YK
There are so many.... But easy to find...
Head codes are the same for both 389 and 421 for 64 65 66
just different combs for each... But you most likely won't find a 64 543796 or 65 #75 on a 421 block
Source(s):
65 ... 421 HO .... YK in my Green 69 4spd car....
67 ... 428 HD ... YH in my Blue 69
4 400's 2 455's 1 LT4