Sorry, I guess my post sounded harsher than I intended it to. I wanted to send a strong message because in the past I have broken many things by blindly charging ahead when I didn't know what I was doing. All I am saying is that you would be better served to stop working on the van until you have a factory service manual and can sit down and read about what it is you are working on.
You are obviously enthusiast and have some experience working on cars, but the things you pointed out are items that are fairly basic on Dodge vans. The Thermoquad is a somewhat complicated carburetor and definitely not something that should be messed with lightly. I am cautioning you because your van seems to be largely intact and in original condition, and I would hate to see you damage something by mistake and regret it later.
I strongly recommend NOT getting a rebuilt thermoquad unless you get it from http://www.thermoquads.com/
You can likely rebuild the Thermoquad yourself if you get a factory service manual, go slow, and follow the directions. Don't just spray downt he outside with carburetor cleaner. You need to remove the carburtor from the vehicle, disassembly it, clean it inside and out, blow out al the internal passages with compressed air, reassemble with all new gaskets, and adjust all the settings before reinstalling the carb on the motor.
You definitely need to get the idle speed down to the stock setting. The exhaust crossover is a passage in the intake manifold that has exhaust gasses routed through it from one head to the other. The purpose of the crossover is to heat up the intake quickly to aid in vaporizing fuel and improve fuel economy and driveability when the motor is cold. The choke on the carburetor is opened partly by a bi-metallic spring mounted in the intake manifold that is heated by the heat form the exhaust gasses. If the exhaust crossover in the intake manifold is clogged (which can happen) fuel economy suffers and emissions go up.
You also need to take the advice posted before and verify all the vacuum hoses are intact and hooked up to the right places. There should be a vacuum diaphragm under the hood and a factory service manual will have vacuum hose routing diagrams.
As far as the OSAC valve is concerned, most people bypass them to improve performance and economy. Since you are in California, you should probably hook it up at least for the emissions inspection, but I would recommend leaving it bypassed for daily driving.
The purpose of the spring you mention is to pull the transmission kickdown linkage forward tight against the throttle stud on the carburetor. If the spring on there doesn't seem to be working, I would first make sure the kickdown linkage is attached to the arm on the transmission, then adjust the kickdown linkage as per the factory service manual, then, if the kickdown linkage still doesn't return forward on its own, replace the spring with one of greater tension. The transmission actually has an internal spring on the valve body that is supposed to return the kickdown linkage forward. The big spring up by the carb is there to take up slop. If the linkage can be pushed back and it stays there, then it is likely either disconnected or there is a problem in the transmission.
As you dive into this project, I will warn you that most remanufactured parts such as carburetors and distributors you purchase from parts stores these days are junk. The assembly line warehouses in China that rebuild these items disassemble them, throw all the parts in a big bin, clean the parts in solvents that strip the factory applied protective coatings off the parts, then reassemble the items with no regard to matching the internal components with the calibration of the carb or distributor. The end result is a part that works poorly at best. whenever possible you should purchased good condition used original parts, new old stock parts, or take the time to find American companies that still take the time to make sure their parts fit and work right.
Another thing to avoid is the temptation to buy "high-performance" items such as the Mopar Performance distributor. Most high-performance items are designed to be used on lightweight rag racing cars. For example, the Mopar Performance distributor comes with very light advance springs that are completely unsuitable for any street driven vehicle, much less a big heavy van. You van will do just fine with regular replacement items.
Ask questions as you go. I promise I won't bite your head off again. i just wanted to make sure you knew how important it is to get a factory service manual for your van and to USE it. I just want you to not make the same mistake I have made. In fact, HERE'S
a factory service manual for your van for $34 right now on eBay. It will be the best $34 you will ever spend on your van.
Good luck, and keep us posted.