I hope this is post is in the right section.

I have a 2000 E350 step van with a 5.4l engine dually and it's 16ft long from bulk head to rear and it's a utilimaster body that I use as a service truck when not at work and also would like to tow a small tailed with it. I was doing some research and I couldn't really find to much info on converting it over to diesel, but I didn't want to do a regular 7.3 conversion. I have looked at a few engines like cat and international that might fit and offer some reliability. If any as any info on this topic I would gladly take it.

Ps. I've had this step van for about 5 1/2 years and it's needed very little repairs and is 4 1/2 out of 5 for reliability so far and I have put custom shelves and various other things to outfit it to use so that why I don't want to get rid of it to by one with a diesel.

 

Vans are a nightmare for conversions. I have done them, I will not do another. A 7.3 wouldn't be as bad, aside from a complete lack of any sort of space in the engine compartment. You'll have to build a harness for it, or at least modify the truck harness to get the plugs into a useable spot, and allow it to interface with the van's existing system.
A 6.0 will take a ton of custom parts as there is a lot of stuff that is van-specific with that engine, and it is no longer available.
I have seen people use the Cummins engine, one was in an RV with a five speed behind it.
I have a van with a 7.3IDI currently, that is scheduled to get a bunch of work done here soon including adding a turbo and going from a C6 to an E4OD transmission.

 

I will post pictures of engine compartment later today but some of them come with various diesel engines most being the cummins 5.9 . In the inside of mine the dogame house of the engine and floor plate over the Trans is removable, which leaves the length of the entire bulk head open for work space (aprox 4ftL by 2 1/2 to 3ft W)(3ft W over the dog house) but again I will be going to take pictures and messurements of exacts. I have access to 3 different complete donor vehicles running and drivable just recked, all are straight vans one with a dt466, isc 8.3, I think dt360 is the other. All are in line 6

 

those engines are going to be a bear to get in and running.
your best bet is to look for a wrecked 7.3 powerstroke powered van to use as a donor.
this will give you everything you will need to do the swap in one package.