The '66 blocks are set up for cross bolting on mains 1-6, the later blocks omit a couple of the casted bosses. Also both the main and head bolts are all 1/2" bolts instead of 7/16th.
'66 block. John DeLorean had big dreams at one point.
'67-'69 blocks.
The crank just barely dips below the block. This is a '69 250.
