If the van has a 4-speed transmission, and you are planning a swap anyways, you could probably take the trans from the van (unless it's electronic shift, of course). If the van has non-electronic shift (don't know what year the trucks shifted over to the 4L60E), all you need to do is swap the tailshaft housing and speedo gears from your existing trans to that one so you have proper speedo reading and you can bolt up your torque arm.
The V8 requires replacement of the frame mounts and front springs (you could keep the V6 springs but they have different spring rates and may cause handling problems, although it would be as though you installed a set of lowering springs). And then there is the wiring issue... The same goes for the 3.8 and 4.3 V6 engines.
The only direct swaps are F-body 3.1 and 3.4 engines for the 2.8. However, with either, you will also need to replace your flexplate (your 2.8 is externally balanced and the others are internal... you'll shake the crank right out of the bottom end) and some of your accessories (or just the pullies) if you have v-belt driven accessories (keep the brackets on the replacement engine). With either, you'll more than likely need to also replace the water pump or you'll end up replacing it when it's least convenient because the seals have dried out and cracked. Oh, and if you don't have AIR on your car right now, you'll want to swap your passenger's side exhaust manifold so you won't have to deal with the AIR divert valve and fitting (mine keeps coming loose and leaking

). The newer engine won't have the idle air pipe connected to the throttle body, so you'll need to find a way to connect up the large hose coming from the charcoal canister (there is a large fitting on the TB vacuum block and there should be a pipe with the replacement engine). You also will notice the new engine doesn't have the seventh injector... swapping is not required, but it may cause hard starts in colder weather. You'll also need to replace the wiring connectors for the TPS, IAC, and oil pressure unit, but the rest of your harness swaps over. There is no auxillary fan switch in the passenger's side head, either.
Performance: 86 2.8 is rated 135H and 160 TQ, the 3.1 140H 180 TQ, and I believe the 3.4 is 160H (don't know the torque spec).