As previously inferred, since that piston is a Super Stock replacement, I am pretty sure that it mirrors the compression ratio of your stock piston which would be 9.0 with the 75.4 cc head.
As to rear end ratio, there are simple means to determine the ratio both with the pumpkin "in" and "out" of the car.
1. If the rear end is still in the car, you will need to jack up the rear, maybe by using jack stands. Mark one of the rear tires with a piece of chalk at the 12 o'clock position. You will also need to put a horizontal line on the drive shaft. Rotate the tire one full revolution while watching how many times the drive shaft goes around. If it goes around 3 1/2 times (and you have a posi in the rear), that would denote a 3.50:1 gear. If it goes around 3 3/4 times (again with a posi), you probably have something like a 3.73:1 gear.
If you do not have a posi, all you have to do is multiply the revolutions by 2. For example, if the driveshaft rotates 1 1/2 times when you you make one revolution with the tire, you multiply that by 2 which will yield a 3.00:1 open rear gear.
2. If the rear end is out of the car, you simply have to count the teeth on the ring gear and the pinion gear and divide the pinion gear number into the ring gear number.
For example, if your ring gear has 37 teeth and your pinion gear has 9 teeth, the result would be a 4.11:1 gear ratio.
If the gears are still in the housing, you can usually count the respective teeth by placing a horizontal chalk mark on the ring gear and rotating the gear to determine the number. You would do the same thing on the pinion gear. Then do the division math as cited above.
Hope this helps!
BT
