'77 W series gearbox will not bolt up to a 3SG-. Your '77 probably has a W50, or if lucky a W57 with modded g'box x member, or if youre real lucky a P50. The 3SG requires a W55/W57, or W58. You will need a 2SC bellhousing to fit any of these boxes to a 3SGE/3SGTE.