I have been dealing with undocumented vehicles since about 1985, over the years it has gotten very difficult.
I build my 'Case" before I go to the DMV.. Different states have different agencies for vehicle registration/licensing.
I start off by looking the serial/Vin number up in the Standard Catalog of American Cars. If you don't have one, buy one.. Barnes & Noble usually carry then, if not they will order one for you.
Once you find the page in the Catalog that has the info for your car, copy that page and the cover of the catalog. This will provide written proof regarding the pedigree, location, etc., of the serial number. In the case of the EFV8's and other vehicles the correct location of the serial will be documented.
The requirement to certify the serial number varies from state to state. find out what your state requirement is and go accordingly. Some states will allow any peace officer to do the inspection, some states/counties will send an officer to your home, CA requires the vehicle be transported to the DMV or the CHP office .