Here's a link to a thread here on the forum for changing from what you have to the block mounted starter nose cone. So obviously the reverse must be possible. from what I have read (considerable) on this subject, any gm starter will fit either the 168 or the 153 tooth flywheel. According to the source the bendix gears are the same. Easy enough to verify, go to the parts store and match the part no. for either starter.
http://67-72chevytrucks.com/vboard/s...d.php?t=363843
If you have the block drilled holes then I would just get a good delco or powermaster ministarter and bolt it up and solve this problem.
Just for S and Grins I'm going out to the garage and see if I have a bell housing mount nose cone and see if it will mount on the block bolt starter. I have a few of those.