The used market is much cheaper than that. I just looked up one website that priced the engines for 900-1400, and talked to a couple local yards that will pull the engine, tranny and computer for around $2000. Try here:
http://www.lkqcorp.com/index.aspx
The big block will fit easily enough. The 2wd trucks had them as an option for a couple years at least. Original big block motor mounts towers are kind of expensive, but not really required. You can use the small block towers. They can also be moved forward a few inches for more room, there are holes already in the frame. Some guys just hammer the firewall in one spot where clearance is tight to make room and leave the mounts in the rear position.
Since the truck has been stored outside for so long, I recommend cleaning out the kick panel vents as soon as possible. Also might want to pull the back panels off the seats to make sure no rodent nests are there. Check the latest page of my jimmy build to understand why.