IM's come in different flavours.
IM's early units in California. Side curtains
IM's early Vancouver, then, IM 1984 on full frame no pan Roadster/ Roll up windows.
IM's AC, or IM's 900 series front end, Audi, Subie, and IM-6 cyl P Roll up windows.
Ed, In my mind I would value a 30 year old car differently.
First I would add up all the options it had, engine, tranny etc, then things to be done to bring it to a reasonable condition, not necessarily to complete redone, and then take that from the price I would think a 30 year old car should bring. Hence most times I stay away from them as I know they are a money pit.
I would be interested in @Alan Merklin's thoughts on this one.