Look for a coworker who might have the same configuration in a truck or van.

Remember this:

1. If heater core nipples are same size, attach the hose from Thermostat area
to BOTTOM nipple - otherwise air pocket in core

2. Use a flow restrictor in the above line - avail at dealer

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.