[spacemanspif]

Used complete engines are always a crap shoot when bought off CL.com. You would be much better off buying one from a junk yard that comes with a warranty. If you are going to need this van up and running in 4 months, you are better off not doing an engine swap because they ALWAYS cost more than you expect and take more time to complete. Your best bet right now is get rid of the TH350 or 400 (3spd trans) and get an overdrive trans (200r4 or 700r4). I'm guessing later versions of the van came with a 700 so you should be able to get the trans, crossmember and proper length driveshaft (if needed, I forget the different lengths of transmissions and if crossmembers play well with them all) from a junk yard. Another option is a manual swap but pedals and hardware placement might be problematic with those stupid engine bays. Getting the engine running properly is most important, do a tune up and compression check to see how the engine is overall and then start tweeking, a wide band o2 is a great tuning tool if you can afford it. The carb may need a rebuild as well. That thing should be good for close to 20mpg on the highway but will never be a sipper, diesel will probably only net you 22mpg...

Carbon footprint is a difficult one that's all on you, but at least you are keeping an old car running instead of adding the cost of recycling it and building a new one...