IMO 3.55's are a huge waste of time/money, not to mention 3.27's
You can find a set of 3.73 gears in alot of full size F150 pickups, and alot had 3.55's too.
The way you can tell what the gear is in the rear is by doing this:
Jack up the rear of the car so both wheels are off the ground. Put car in neutral.
Then mark a spot on both the wheel and the driveshaft.
Count how many times the driveshaft turns for ONE revolution of the wheel. This is the gear ratio. with a 3.55:1 rearend, the driveshaft will turn aproxiately 3 and 1/2 turns for one turn of the wheel. But make sure that the wheels are turning in opposite directions and at the same speed when turning one of them, because if not, then you wont get an accurate ratio. (if this is the case, turn the driveshaft by hand and count how many revs you turn it while someone is watching the wheel)
or you could always remove the differential cover and count the teeth on the ring and pinion gears. divide the # of teeth on the ring gear by the teeth on the pinion to get the ratio.
Or do as Agent_4573 suggested