Two manual trans available in 84. Most common was the M19 with a final drive of 4:10/1. Also available was the MY8 with a final drive of 3.32/1. The cases of both earlier transmissions had no ribbing for reinforcement. Later four speeds (85-86) (M17) has extensive ribbing for strength and featured a 3.65/1 final drive. If you have the car the trans came from, the RPO sticker on the left front wheel well will have M17 or MY8 in the codes.
No sticker? Put it in 4th and count the revolutions. Due the the fourth gear OD, revolution counts will be 2.42/1 for the MY8, and 3.18/1 for the M19 The later model revolution count will be 2.96/1.