Welcome,
As far as I know the GMC 450 with a 3-71 diesel was made in 52 with some sold in/as 53. I don't recall production numbers if they exist.
The air box heater you're thinking of was used on other inline 71 series in various applications, parts made for or by Detroit/GM Diesel. If hunting parts my first call would be to Leid Diesel Service in PA, 717-776-7725 Or In Michigan at 616-754-5871. They also have a location in Wisconsin, ask the others for a phone number. A good resource for all your Detroit 2 cycle needs.
The more practical solution is a plug in block heater, much less trouble and expense, and readily available new.
Grigg
