there is no real gain between the 7.3 super duty intercooler and the 6.0 intercooler.
if you want to put a intercooler in the easiest way is to get all the parts from a early 99 super duty.
you will need the intake runners, intake manifold(spyder), intercooler pipes and the intercooler. its gonna cost you more than 300 most likely.
No need for the intake runners (plenums). OBS and E99 7.3's use the same 2" plenums. L99 changed to a 3" plenum.
No pipes, regardless of year are going to be a direct drop in to intercool an OBS truck. You can use an E99 intake spider and 7.3 intercooler, but the tubing is still going to need to be cut/welded to fit. Get whatever pipes are the cheapest, and modify them to work. There is also a guy who will fabricate a set of pipes for you, based around the E99 spider and either a 6.oh or 7.3 intercooler for $200 shipped to your door. I can post his email if interested.
You can use a 6.oh intercooler or 7.3 in the OBS, doesn't really matter. 6.oh is a slightly more efficient cooler than the 7.3, with the drawback being it has plastic end caps. If you are running big boost numbers this is a no-go.
You are going to be hard pressed to put an intercooler together for $300.
E99 Spider : $100-$150
Tubing : $100-$200 (depending if you modify it yourself, or buy it as a kit)
Intercooler: $150-$300 (depending on condition. Find one locally, shipping an intercooler aint cheap)
Boots & clamps: $50-$150
Even if you pay bottom dollar for all your components you're still looking at $400. I'd say $550 or so is the average.