I saw a vid on YouTube a while back where someone removed right front fender and installed on an internal structure (firewall?). Can't find it now, but may be worth a search
Not sure if that helps.
Thank you! Yes, I think I saw the same video. They installed the coolant heater in a Ford F550 truck under the right fender
You can just see the exhaust side of the heater in this image. It's on the driver's side, just aft of the front wheel. I've had it there for about 15 years. No problems, works perfectly - even in the mountains of Colorado, as well as the upper Midwest winters.
The unit is mounted under the van, near driver door.
It is connected to the coolant lines that run near there from the factory (in case you want to heat to the rear of the van with the coolant).