In addition to what Paul mentioned I can contribute the following:
The last of the B18s were also fitted with the aluminum bellhousing bracket
which took the place of the sheetmetal flywheel cover on the earlier models,
giving reinforcement to the lower part of the bellhousing. These also had the
alternator mount that the B20s had on the right front.

Later 6-bolt (pre-74) blocks had a similar mount off the timing housing for
mounting an alternator on a right hand drive car on the left side or a power
steering pump. I think all had the 3 bolt holes on the right to mount the old
generator bracket also.

I had to replace an engine block in Panama and the only one I could find was
an 8-bolt block, but it was bare. I was successful in installing the complete
6-bolt crank, rods, etc in that block with relatively minor problems, although
I was slow in resolving them. In the 6-bolt block all of the mounting holes
are inch sizes, with the main cap bolts being ½"-13 UNC and the bellhousing
bolts being 7/16"-14 UNC. On the 8-bolt block everything else is unified threads
but the maincaps and the bellhousing are both retained by M12-1.75 high strength
bolts. Once these bolts are replaced with M12s, the rest of the assembly is
straightforward. Note that the M12 bolts are about 0.475" in diameter, about
midway between the .500" 1/2-13 UNC and the .438" 7/16-14.
The headbolt holes of the 8-bolt block are still 7/16"-14 UNC.

Once the engine was assembled it worked well for a number of years until the
oil pump went bad with the auxiliary shaft working out.
--
George Downs Bartlesville, Heart of the USA!