If you want the easiest swap possible using stock parts here's what you can do:
Get a 5.8L out of a truck, bronco or van (fuel injected).
I believe you can use the stock lower 5.8L intake manifold with the mustang upper intake manifold (the width of the lifter valley is different between the 302/351), but i'm not sure if the runner diameters are the same or similar. Some port matching would fix that, or buying a specific 5.8L swap intake manifold.
You'll NEED to get a special dual sump oil pan/pickup for the 351, reverse flow water pump, special accessory brackets (for serpentine belt to run the accessories because bolt holes line up differently in 302/351) and swap headers. You'll get alot more lowend torque and possibly close to 1/2 second less in the 1/4 mile.
the stock T5 for your year is rated for like 300ft/lbs of torque, but depending on how you treat it can last quite a while. Unless you plan on taking it to the dragstip and launching it at 5,000rpm with slicks i wouldn't expect the T5 to suddenly grenade.
Good luck!
------------------
'84 Mustang 5.0 LX
My car