I found that it is important to keep the gaskets and seals from getting squeezed out as you tighten things down. The one product that I have found to work the best is Gasgacinch( http://www.cbperformance.com/catalog.asp?ProductID=1074 ). This stuff works just like contact cement and must dry to the touch on both surfaces to be put together. It will keep the gaskets in place and throughout the life of the engine, keep the seal from "oozing" out like the silicon products will do.
With the thicker gaskets on the end of the GMC oilpans, if you put a LIGHT coat of Permatex Form-a-Gasket.( http://ken-co.com/detail.asp?PRODUCT_ID=3D )on the surface that the pan contacts, the pan will slide nice on the gasket and not try to push the gasket out of the way and still seal up nicely. The Gasgacinch can be bought at your local motorcycle repair shop (these guys use it to put engine cases together) and the Form-a-gasket can be found at the local auto parts store.
Hope this helps.

"The Operation of GMC trucks is normally smooth and steady, with no loud rattling, knocking or unusual noises. A good driver will quickly become accustomed to driving a GMC truck and will get the feel of his vehicle regardless of his engineering knowledge."-TM10-1563