Sorry about not reading the first post very carefully (it is a very clear posting/question).
Hagerty will most likely have problems insuring the old vehicle because the insured-person does not have another insured vehicle. That will be the case with most other insurers of antique vehicles.
The only other option, as suggested by others above, it to try to get a regular insurance policy with an agreed upon replacement-value. GEICO, State Farm, Allstate, USAA, . . . ??
