[Zero10]

clutch, oil pan, front end, hell, drop the engine.
It takes about 5 hours to have it out of the car and another 5-10 to put it back in. Beats trying to do the oil pan gasket in place and you have a way better chance of it not leaking if you can see what you are doing (Read: changing it with the engine out of the car). The time difference between doing the work with the engine in vs out is negligible, and the frustration factor is far lower.

The secret to dropping the engine quickly is to drop the front suspension with it. Undo the 4 nuts on the strut tower, hang the brake calipers from above (unbolt from spindles), drop rear control arm mounts, unhook rad hoses and remove rad. Remove the bolt in the intermediate shaft and separate it, pull the DME wiring through the firewall (sounds hard, but really isn't), and un-do the power steering lines. If your A/C is in good working order then hang the compressor to the side, otherwise just remove the lines on it so you can drop it with the engine. Pull the battery wiring off and remove the distributor cap.

You'll have to do so much of this to change the oil pan gasket and water pump that you might as well just drop the engine. Plus, you can count on both engines needing a clutch. Given the miles on them you'd be crazy not to change it while you have the chance.

*EDIT*
Your car is an 87, this means it has an oil level sensor in the oil pan. Your spare engine is an 86, this means it does not. This means it has a different connector on the car side for the engine harness. Your car will have a black 8-pin connector by the DME plug, the 86 engine will have a red 4-pin connector. Best bet is to swap the oil pan and wiring harness onto your 86 motor, but plan B is to ground the white/yellow wire in the 8-pin connector on the car side, then make up some jumpers from the red 4-pin to the black 8-pin, this way your MPG gauge and tach will work properly.