The 87 changed the coolant tubes.
On the 86, your heater hoses are hooked up this way.
One goes to the Filler neck.
The other sits inside a channel in the frame under the battery tray and runs up and around to the front of the trunkwall inside the engine compartment. From there, it runs to the connection on the waterpump.
The 87 does NOT have that back hose. The hose runs to the passenger side coolant tube and feeds in on an inlet there.
The 86 has the connection on the waterpump, the 87 does not.
If you have an 86 engine in an 87, you have a couple of options...
#1. Buy an 87 Waterpump and replace the 86 one.
#2. Cap off the port on the waterpump itself (The tube unscrews, but I am not sure what size threads or plug you would need) with either a plug, or a rubber cap designed for cooling systems and high pressure.
#3. Cap off the coolant line on the passenger side where the Heater hose connects, and run an extension up and around.
To be honest, your best bet is #1. Difficult to change a pump in the car, but would cause the least amount of headaches, and would eliminate the need to worry about any potential "Redneck Engineering" problems that might occur.
I recently installed an 87 motor into my 86 SE and installed a new waterpump for an 86 onto the 87 motor. Had to have it so my heater worked.