I get there might be a need to slightly shim things. A 3/4" gap is not shimming IMHO.I know different car, but my 70 from the factory has washers under the core support to set the height that was needed and then used the bushings.
My initial assumption was there must be different bushings depending on which model & this group could/would confirm the specifics. Seems it's still a mystery. At this point, I'll probably just make some spacers & go from there.