Just to help those in the audience, Kitbashing is an old model railroading term which involves Taking one or more model railroad kits (often structure kits) and changing the construction process or combining parts to make a unique model.

My answer May not be what you want, but I did find a diecast FDNY support vehicle made from a utilmaster. It is a completed work, so you may not have all the fun you desired. BUT, if you contact the seller, he may be able to give you his supplier. Good luck.

Also, I found a place called rudytoys.com that sells diecast (with plastic parts) support vehicles at a pretty decent price of 4 for $20. They advertise that they special in finding hard to find diecast toys.