[91 Mustang LX]

ok to start things off im new to this board and so far the help has been amazing. ne ways moving on i own a 1991 mustang LX 5.0 HO. ive got so many questions so il try to make this as short as possible.
ok what kind of gears should i get for my car is 3.73's still street reliable? wut is the fattest tire i can put on my 16 inch pony rims and does ne one have ne recomandations of brand? Is a cold air intake worth it the reason i ask is because im only 18 so im on a limited budget and a 140 on a part isnt all to crazy and finally does ne one have ne sugestions to help me make car run awesome on a budget? thanks