We do all our own clutches, and those chevy trucks with a 366 and a manual transmission are not too hard to do. We have found a rebuilder of clutch plates that sells parts to many of the other dealers in town, so we are getting better parts at wholesale prices.
We recently did our GMC 6000; it cost $477.49 (in parts only, not counting our labor) for a "button" clutch disk, rebuilt pressure plate, flywheel resurface, throwout bearing, and flywheel bearing. The button clutch is more durable, but it also costs considerably more than a standard disk. So far as I know, the guys left the rear main seal alone.
When you go to install the clutch again, have some extra long bolts sized to screw into the block to align the bell housing. Cut the heads off, and then slide the bell housing down the bolts onto the back of the engine. Once into position, you can screw out the alignment bolts and put in the proper installation bolts.