In 1997 the Explorer started using GT-40"P" heads. The spark plug was repositioned more towards the center of the combustion chamber for better mixture burn. You would need a set of "P" specific headers.

You can put heads & intake on a speed density car. Cams are where you have to be careful. There are cams that can also work with speed density, but you can't go past a certain amount of duration and lobe angle separation (I can't remember specific numbers at the moment).

It doesn't matter how much newer the motor is compared to your car. You would only have to swap your timing chain cover, water pump, oil pan, oil screen, balancer, whatever other Foxbody brackets you have in your car, distributor, and your flywheel or flexplate to the newer motor.

Assuming the newer motor has the throttle body intake tract you will have to remove that to take the spacer/elbow off. All your existing wiring will hook right up after having swapped the motor into the car.

For mass air conversion do a search on all the different Mustang sites because that topic has been beaten to death so many millions of times.

After you're done with the swap you have to sign the title to your 86 GT over to me free of charge.