Yeah, the bolt at the top is for the elusive heavy duty tranny.
I used to have piles of Saginaw 3 and 4 spd trannys around here,bought them when they were unwanted,then all the Muncies got expensive,sold all the Sags.Like the man above said,the trucks generally had the 2.85 first gear,the one with no marks on the input shaft.3 spd Malibu types with 6's usually had the 3.50 first gear 3pd.Some Vegas used the 3.50 first gear 4 spd,V-6 Buick and Olds Vega types,used the 3.11 first gear and the V-8's used the 2.85 and 2.54 fist gear.There were exceptions to this,options on ratios .All the Saginaws I ever saw were the same length ,21 inches,all the same input and out put splines.Minor non functional differences like back up light switches,high gear swiches for emmission controls and tailhousing bosses for torque beams and or counter weights can give you a clue to the trannys original application.
The front bearings in Saginaws can get sloppy with age and abuse,then the main shaft bearing surface will become worn where it fits into the needle bearing of the input shaft.The spring washer between the main case rear bearing and first gear can fail,causing the gear to bang around.
All Saginaws,3 and 4 spd,except for over drive models,have the same main shaft and within a particular ratio group ,the same gears except for the counter gear which is different from 3 and 4 spd.The 3spd cases look identical to the 4 spd case,but the boss retaining the reverse gear shaft is different.The gears themselves have no idea what engine is in front,no one model is stronger than another.
Just pick one that suits your needs.
