he easiest,fastest and probably cheapest way of doing this is by purchasing a wrecked donor Mustang with a 5.0
It will take you a lot of time and money if you have to locate the parts separately.
5.0 engine and transmission
All underhood and computer efi wiring harnesses
Vacuum canister assembly-inside pass side fenderwell
Vacuum tree on firewall
V8 fuel lines from tank to engine-4 cylinder lines are on the wrong side of the car
8.8 rearend-drum to drum the 4 cylinder has a 7.5 which won't hold up to a 5.0 engine
The gas tank and 88lph fuel pump is identical between the 4 banger and 5.0 so keep yours in place
You can keep your current springs in place but the 5.0 engine will lower the front of the car due to the additional weight.Its like having free lowering springs installed.
If you've got an auto,its best to use the 5.0 auto transmission so you won't have to worry about swapping to a manual pedal assembly, etc.
The 4 cylinder radiator is identical to the 5.0 radiator
You'll also need the throttle cable/bracket and transmission cable from a 5.0 that connects to the throttle body.The 4 cylinder cable is too short.
5.0 motor mounts
The link below shows some other parts that will be needed.
Changing a Fox Mustang from a Four Cylinder to a V-8 - 5.0 Mustang and Super Fords Magazine