What reason is your commercial business insurance company giving you for not wanting to cover the truck?
Call their competition and say "we have a business and part of that business is we use an antique truck, are you interested in giving us a quote"...see what they say!
Mike B

We don't actually carry commercial business insurance right now. We have a small farm and sell value added products, we just have product liability insurance through the soapmakers guild. Maybe it's time to see about business insurance. Our car insurance company quoted almost $200/month to insure the truck if we want our logo on the side. $50/month if we don't use if for business. Seemed steep to me. I might try country financial, I've had all my insurance through the same outfit for almost a decade, it might be time to shop around.