just looking at the hose connections, do you have the pressure line connected to the right port?
scroll down on this link and see possibly the same set up with the hoses reversed, pressure line is closest to the input shaft. do you have the instructions for the install, do they mark inlet and outlet?
saginaw boxes usually have the inlet port furthest away from the firewall but not sure about the 400 series boxes. there is usually a check valve in the outlet port so not sure if it would even work if the lines were wrong. anyway, just something I noticed on another site.
https://talk.classicparts.com/thread...00-truck.6832/