Look near or behind the alternator. The valve that controls flow of coolant to the heater core on my 1987 Aerostar is located on the left wall of the engine compartment. I imagine the 1991 is similar. Trace the heater hoses from the engine/water pump and you will find it.
Bolts might come loose from the transmission pan while you are driving your 1987 Ford Aerostar because they have not been torqued to specified tightness. They might also come loose because the bolts are not the right size or are worn.
On my 1995 Ford Explorer , to lower the underbody spare tire , I open the liftgate and the access hole is located above the rear bumper at the center of the bumper ( I assume your Ford Aerostar would be similar )
No way man. Unless you own a cutting torch and a welder there is no way your going to be putting in a small block Ford into an Aerostar. Not to mention all the fun your going to have swapping in a different wiring harness and computer.
The relay for a 1994 Ford Aerostar is located to the right of the steering column under the dash area. There are 4 wires coming in and out of it. The colors of the wires are normally 2 yellow/light blue, 1 dark blue and 1 yellow/light green.