Before you make any rash and expensive decisions, wait and hear what the mechanic tells you broke. It definitely sounds like an overheating situation. It might be something as simple as a blown head gasket, leaky radiator, or blown radiator hose or thermostat housing gasket. The oil pressure is concerning, but it is odd that it it varied with the throttle. The "coins rattling in a coffee can" noise might have been pinging from preignition due to overheating.
If the motor was still running, then you had compression in at least some of the cylinders. The short block might actually be OK.
Due to its smaller displacement, a slant six should be built to match its intended use. If your goal is to use it in a van that will pull a trailer, you need to make sure you build it to maximize horspower and torque in the 2000-2800 RPM range. You also need to have the right gears in the back. If you are going to pull a loaded 5x8 trailer then I recommend 3.55 rear axle gear at the highest, and 3.7s or 3.9s would be better. Remember, Dodge put the slant in six-ton dump trucks back in the 60s, they just didn't go very fast. The slant six can do the job if the gearing is right.
My recommendation is to wait and hear what the mechanic finds. If it is something simple like a blown head gasket or a tooth jumped on the timing chain, just fix that and keep on trucking.
However, if you do have to rebuild the motor, I recommend the following:
(1) upgrade to a two barrel carb. The factory two barrel carb package (the "Super Six") used a Carter BBD or a Holley 2280. This is a great upgrade and will improve power and often economy. However, parts are getting harder to find and more expensive. The intake manifold is the same for all cars, trucks, and vans, but vans had special van-only air cleaners and kickdown linkages. The doghouse required a special offset air cleaner and a different kickdown linkage to clear the van trans shift linkage. You can use your stock one-barrel air cleaner and weld in a section from a 318 air cleaner to get the right size hole for the neck of the carb. You can also run a cable style kickdown linkage that can be found on eBay for cheap. I recommend keeping the stock closed element air cleaner but using a factory style cold air intake. Another option would be to get an Offenhauser four barrel intake manifold and run either an adapter plate to use a two barrel carb on it or run a small four barrel carb with the secondaries locked out. You don't want to put too much carb on the motor. Other people have had good success adapting a Motorcraft/Autolite 2100 carb.
(2) upgrade compression and the cam. Ideally, you want to shoot for 8:1 or 8.1:1 DYNAMIC compression ratio, NOT stati compression ratio. usually this is achieved by milling the head. But you have to take the time to do all the necessary measurements and calculations first, and you have to pick your cam before you know how much to mill.
Your 85 slant has a hydraulic flat tappet lifter cam. There are some good aftermarket cam profiles that have been researched by slant six enthusiasts and that work well with a hydraulic slant six cam. Read this thread: http://www.slantsix.org/forum/viewtopic.php?t=43301&highlight=cam+low+rpm+torque
I recommend going with one of the Oregon Cam profiles recommended in that thread and having your stock cam reground by Oregon Cam (unless it is damaged beyond reuse).
(3) upgrade exhaust. This is easy- do it. I recommend you upgrade to dual exhaust using a Dutra front manifold and a modified stock rear manifold. Then run 2 inch true dual exhaust or Y the exhaust together after the manifolds into a 2 1/2 inch system before the free flowing muffler and then 2 1/4 after the muffler.
(4) Upgrade the valves and the head. The best bang-for-the-buck in upgrading a slant six is imrpoving flow through the head. Porting, oversize valves, and gasket matching the head and manifolds is a great way to improve power and economy.
(5) Other machining. Obviously you want to balance the rotating assembly, but I also highly recommend taking the time to improve flow in the oiling system. See here: http://www.slantsix.org/forum/viewtopic.php?t=29492&highlight=deburr+oil+passages
Since your motor has hydraulic lifters, you really want to have the best possible oil flow through the whole oiling system. On hydraulic lifter slants the lifters are fed oil through the hollow rocker arm push tubes. The oil goes form the crankcase, through the pump, through the block, up to the rocker arms ahft via a hole in the rearmost rocker shaft pedestal, through the rocker arm shaft, through the rocker arms, into the pushrods, then down to the lifter. Don't worry about high volume oil pumps, the stock unit is fine. Try and keep your original oil pump and get it rebuilt- aftermarket pumps are of very questionable quality.
Report back what your mechanic tells you and I can give you my opinion on what should be done.