If it has an SM420, it is a later transmission than original and would not have fit the original clutch housing. The original 4 speed transmission would have been spur gears with no synchronizers and it was common practice to install a later transmission and clutch housing. The stud on which the the clutch and brake pivots is pressed in. If there was welding, something probably broke. The drilling on the passenger side of the transmission is for a parking brake if you have an open drive shaft such as is used in larger trucks. If you still have the closed drive shaft, a parking brake behind the transmission will not work. GMC and Chev are the same.