From what I have seen of the 68 blocks I think they were the same casting
(or very nearly the same) as the 69 B20s, with the bellhousing bracket on
the bottom, alternator ear, etc.

The stock alternator on 69 models is mounted with a ½"-13 UNC hex bolt with
a 3/4" head and some have a bushing about ½" thick in there also to get the
right belt alignment. Both Bosch and SEV/Motorola (French) were used and both
used a 3-wire external regulator. Later models used an internal regulator,
probably starting about 1976. If you get a stock alternator the one to get
would be for the 164, either Bosch or SEV but you have to get the right regulator for the brand of alternator you have.
If the tension arm is mounted on the head you may be able to get by without
bending it. I had to grind one of mine because of interference with the blower
on the alternator
--
George Downs Bartlesville, Heart of the USA!