I've used the engine crane method s few times,I had the removeable tranny hump on the floor ,so I took that off (lot of work,every bolt was rusted,and there was a tar like substance that was like a weld I had to chisel loose with a putty knife to get it off!)---but it was easy to get at the bell housing bolts with it removed,that's mainly why I did it that way on my '71 GMC...
On other trucks I used the long extension trick,it worked best on my 77 GMC that had a 3" body lift,the bolts were in plain view..on non-lifted trucks you sometimes have no choice but to climb on top of the engine and use a box end wrench on the top bell housing bolts..
You'll grind all the skin off the back of your hand on the firewall..
GM was so kind to leave a whole INCH of clearance to get at those bolts..
I'd suggest using a 6 point socket,if you use the extensions--last bolt you want to round off are the top most bell housing bolts!..
It was easier lowering the tranny & T-case together as a unit by unbolting the cross member,whether I used the floor jack or the engine hoist...(hoist was much easier,no wrestling the unit out from under the truck)..
I always used long bolts with no heads in the center bell housing holes to align the tranny,it makes it a lot easier,you can usually just shove in the unit with your feet by pushing on the t-case from behind,once you get the two bolts in the tranny aligned..
The TH400 does use a vacuum modulator,you need manifold vacuum,the same port the TH350 was connected to is the best,its behind the carb on the intake..you can use other ports that supply manifold vacuum,but ones on the carb might not be full manifold vacuum or large enough to supply enough to the modulator..
The TH400 does have a solenoid that kicks it down to "passing gear",that is controlled by a switch on the gas pedal bracket..does the same thing as a detent cable on a TH350..only electrically..
It doesn't "have" to be hooked up if you dont want to hunt down the switch,you can just manually downshift it yourself..
