No, the explorer 302 is ACTUALLY a 303!! It has different motor mounts, and can only be used with a Powerglide transmission. Also, the 303 Explorer motor has was too much HP and TQ for a little Mustang. You'd need a full roll cage to support all that mighty power if you ever COULD fit that huge Explorer 303 in there.
You can use the long block from the Explorer -- but oil pan, front cover, accessories, etc. will need to come off of your car. The Explorer/Mountaineer used a big cast aluminum oil pan that won't fit in the Stang without mods.
The accessory drive is different (Explorer stuff sits about 1.5" closer to the engine and has different positions for some items); oil pans are different; Explorer has distributorless ignition that's trigger fired off of a wheel mounted on the balancer; Explorer intake is like the Cobra intake (same lower/similar upper); 65 mm t/b on the Explorer; depending on when it was built in 97 it could have GT40P heads or GT40 irons as opposed to E7's on the HO; externally fed egr (off the header like an SN95 Stang as opposed to internally fed egr on the HO; short block is the same as 93/94/95 HO's - hypereutectic pistons.
The Explorer/Mountaineer motor is basically like a 93/4/5 Cobra motor with a different cam (SUV cam for low end torque), with non-roller 1.6 rockers (like the HO) and a limited exhaust system (restrictive headers and a single exhaust system). So put shorties on it, an HO cam and the 1.7 rollers and you've basically got a Cobra motor.
You can use the long block from the Explorer -- but oil pan, front cover, accessories, etc. will need to come off of your car. The Explorer/Mountaineer used a big cast aluminum oil pan that won't fit in the Stang without mods.