Yes, the th400 was the only auto used in the square body 1 tons until ‘90 (‘91 used a 4l80e).
Fyi, be sure to get the adapter between the trans and tcase if you’re using the th400. The crossmember only fits a 1 ton frame, it won’t work correctly on a 1/2 or 3/4 ton.
Glad I could help!