115 351 32 00 is the diff assembly part number. 115 was the first body style that used that part, 351 is differential assembly / housing 32 and 00 designate the specific part.

On the diff casting near the back cover there is a machined area ( the area for the rear cover gasket ) the left lower or right upper will have some stampings, the ratio should be there.

A quick way is to rotate the drive shaft and count how many times it rakes to turn the axle _one_ revolution. Sometimes ,with a non limited slip diff, it is difficult to keep both axles turning at the same rate. In this case, hold one wheel from turning and count how many drive shaft revolutions it takes to turn the free axle _two_ turns.

It really sounds like the trans isn't starting out in 1st. First is a very deep ratio at 3.68, couple this with your possible 3.58 or 3.69 rear ratio and you will know that you have started out in 1st. First is electrically actuated on non 1st start default trans, perhaps the manual is incorect or a different trans was installed at some point.

With your deep diff ratio and pretty good 2nd and 3rd ratios, you would never miss 1st if you didn't know it had it. First is a " free " ratio that does not have any additional gear train parts over a 3 speed, it just has more valve body parts to make the proper elements engage to generate 1st gear.