The casting # on the block should look something like this for a 68 Bronco or Ford truck:
C8TE-6015-A
The "C" Denotes the decade. 1960's
1st # "8" Denotes the year of the decade
The "T" denotes the carline. T is for "Truck".
The "E" denotes engineering group. "E" is engine. (A part with a "B" would be body. "A" would be chassis.)
6015 is the basic part #
The last letter, "A", denotes the suffix or revision. The "A" would be the original part. "B" would be the first revision, and so on.