You might check your engine & starter against this chart:

http://www.gmpartsdepot.com/sb_flywheel_chart.html

There are starter shims available.  I suggest you remove all cover pans, put
the starter on, and observe it engaging the flywheel from behind.  You may
have to pull the transmission or engine so that you can.   Then shim inner
or outer bolts until it engages correctly.  I would suggest the outer first
as that should move the starter in.   Shimming both only moves the starter
further away.

Check your flywheel.  You may have one slightly undersized, especially if
the previous starters have been grinding the teeth off.

A possibility is that your builder align bored the mains and so the crank
now sits higher in the block.  If so, you may have to have the starter
machined slightly on the mounting pad to raise it up enough to again align
with the center of the flywheel.  However, shimming will probably do it for
you.  Machining should be the last thing to try, as it is the most
expensive, and your starter will be something not easily replaced if that
ever is needed in the future.

A lot of work, yes, but what is your alternative?  Sit and watch the car in
your garage?