Just a week ago, I had the in-dash heater core out, in my '98 Club Wagon, which in many ways resembles an E-150 cargo van. There was a 3/4" tube and a 5/8" tube on my core. The hoses for this were custom shaped, with quick-connect fittings to the heater core. If your heater core tubes have bosses on them, that implies that a quick-connect fitting is expected.
I also have heat in the passenger area, with a radiator along the rear left passenger compartment in this passenger van. Perhaps a true cargo van lacks a heater along the left side of the van, like mine. So my plumbing might be different than yours. But the behind-the-dash heater core that I removed, and re-installed, is served by coolant lines that emanate from the rear of the engine, not the front where the water pump is. They are "complicated," not like the simple heater hoses of yesteryear.
"MasterBlaster"