I may be wrong and others will chime in, but im thinking the v6 cars came with 7.5" rearend ring gears and the v8 cars came 8.8". Im not sure where v6 parts could be purchased. Do you know what the rear ratio is in the car now? Ford mounted a tag to one of the rear diff cover bolts with numbers stamped on it. Get under the car and check it out. Report back.
The 7.5 gear set can be swapped in with no other components. The shop should be competent enough to use the exsisting bearings assuming the rear has no proplems now.
Buy FordRacing proper rearend gear set of your choice size and take to a shop. Roughly $200labor
If the rear gear in your car now is a 2.73, switching to a lower ration (although numerical higher) 3.73 or 4.10 will multiple the torque out put at the rear wheels. Or you could look at it like this....your 1st gear now can top 50mph at redline. Swap in the 4.10 and it will be 25mph. The same applies to the other gearing speeds. The car will feel quicker off the line and the engine load will be less