There's no 39-liter engine in a 1991 Dodge Ram 250 van. The largest engine offered in that year was likely a 5.9L (360 cubic inch) V8. A 39-liter engine would be enormous, more suited to a large marine vessel or industrial application.
To get the correct firing order, you need to specify the engine size (e.g., 3.9L, 5.2L, 5.9L). Once you provide the correct engine displacement, I can look up the firing order for you.
