They are not too difficult, but there are several "tricks" and special tools that make it easier.
Usually doing a refresh, if the trans is in decent shape, the clearances should come out correct. If you start swaping major components, and adding a bunch of HP parts you may need various thickness snap rings and bushings to set the clearances correctly. If you have never rebuilt one before, take some pictures when disassembling the transmission, and some close up photos of how the over-running clutch rollers and springs are assembled. It is easy to install the springs wrong, and most manuals don't show/explain the correct way to install them. The servos can also be a bear to install.
When doing the valve body, set the selector detent ball and spring apart from the rest of the valve body so you don't confuse those parts with the balls and springs inside the valve body.
You will need a bushing driver to replace worn bushings. Before I bought a driver, I just took the parts to a trans shop and had them do it. usually the torque converter hub bushing in the pump and the output shaft bushing in the tail shaft need replacing. Also sometimes the front drum bushing.
You need to compress the front drum spring retainer to replace the seals for the front clutch piston. I made my own tool to do this, but I have also done it using two large C-clamps and a flat peice of plywood under the drum. You should be able to compress the rear servo by hand, but need the large C-clamp to compress the front servo.
As mentioned, removing the front pump, screw in some long 3/8" bolts with large washers into the two threaded pump holes, the slide the "U" shaped part of a prybar along the bolt hitting the washer like a slide hammer. There are other ways to get the pump out, but that is what I use.
For assembly, get some long 5/16" bolts and cut the heads off so you can align the pump and gasket up when reinstalling the pump. This is easiest if the trans is held so the pump is straight up, with the tailshaft pointing down. If you don't have trans assembly lube, vasoline works OK.
