To find your gear ratio installed in a non-overdrive automatic or manual transmission car
verify the speedo is working
get a tachometer
if you are able to go 65mph @ 2600rpm then you have about a 2.8 or 3.0 rear end ratio
if it is more like 3300rpm@65mph then you are already running a lower (lower is more acceleration, less top speed) end gear ratio
To buy new gears check out any ford performance store.
www.jegs.com or a well know mustang parts dealer are your best bets. jegs has better prices though.
If you wish to port and polish (P&P) then get a chiltons manual and learn how to take out the heads so the shop doesnt charge the extra money. Shops charge between $200-400. Rice shops try to sell porting in "stages" in which each stage will cost you more money. The difference to the shop for each stage is not usualy worth the difference in price. Make sure they can give you a flow bench test on your heads, otherwise they can charge you and not do anything - another attribute of the rice shops.
If you could do it yourself a port and polish kit only costs $40, but you will have to know what you are doing.- you will also have to buy new gaskets and of course oil.
The weakest point of a stock 302 (performance wise) is the flow rate of the intake and exhuast runner in the heads. fix that problem and other changes to the engine will make more power then they would on a stock engine.