jwheels . . . is your truck 4wd or 2wd?
In either case, you will need a flex plate for your engine to bolt the torque converter too. Also - some (all?) 400 small blocks were externally balanced and care should be given to ensure that you have the proper flex plate that has the appropriate balance for that motor.
The other items that will need attention are the cross member to support the tailshaft of the transmission and the driveline (did you get the rear transmission yoke with the deal?)
Seems like the T400 has a solenoid instead of a TV cable going to the carburetor linkage. You will need a throttle position switch to wire into the transmission to tell it when to downshift when you lay your foot into it. You will also obviously need to hook up a transmission shifter / cable - Lokar or B&M make all manner of automatic transmission shifter solutions for your rig - or you could rob one from one of hundreds of hulks sitting in wrecking yards. While you are in the wrecking yard - check the driveline and cross members too.
I am sure I have probably missed a few things or perhaps steered a detail or two into the ditch, but you get the idea. The T400 is a bullet proof tranny if you keep it cool and the fluid clean. Have fun!
