If your pump is in the tank you have two ways to go.
1: you need a new supply line. There are some built for this from the heater companys
....somethink like this:
or this is easier to install. direkt into the fuel filler line
2: you can take a "TEE" as shown befor and install it into the return line of your van.
thats the easiest job, but the problem is that your heater maybe will only work for 1 hour or so.
Some heaters have there own reservoir for the fuel which is filled up during driving. In that case you also can make a "TEE" behind the pump. Otherwise i think the pump from the heater will not be able to suck fuel though the pump.
If you install a "TEE" yes just cut the metal line and rubber hoses each side. You should fix it well with clamps because if it is able to suck air throuh it you will get problems.
BTW: my 7.3 from 2001 has an external fuel pump ???? And there i took the "TEE" between tank and pump.
Soory guys for my bad english....i am german ;-)