My only alternatives seem to be moving cast iron waste pipe (have access in
basement) back a few inches)

Probably your best option is to reset the critter to the standard 12" from the
wall. Another option for even more clearance is to use a close coupled toilet.
Most old fashioned plumbing houses (where the pros shop) will have the 10" type
toilets. Logically having a good 4" extra would solve your problem. Go for a
quality Kohler unit since their close coupled water savers work great on the
first flush (I've got one in fancy Mexican sand color). The better performance
seems to be related to the design and good glazing of the porcelain. HTH

Joe