John,
Your shop is correct, classic Morgan wheels are not hub-centric AND neither are the front or rear hubs. Assuming you get the wheels spun up in a lathe to machine a centralised hole and then then buy a hub centric spacer to reduce the internal diameter of the machined bore what exactly will that mate to? Nothing, because there is not a machined male lip for that purpose on either the front or rear hubs.
At the risk of sounding disrespectful, you may do better to find a different shop that doesn't want (to literally) re-invent the (Morgan) wheel.
Providing you have OEM wheel nuts and tighten them down in a 'star' pattern the wheel will centre correctly.
