Gerry,
Welcome to the group, family, cult, asylum....
If you have not already, plan to join GMCMI and look up the local owner's chapter. If you do not have all the owner and service documentation, it can be downloaded at GMCMI. Do that right away.
Next important thing is to build a good sigfile. As you must already know, not all GMCs are alike and some even OE differences matter a lot. With a '78 (403 engine) and Transmode (minimal interior documentation), you will have some difficult times. There may be others with upfits by the same people that can assist you. To this end, the sigfile will forestall a lot of confusion.
The biggest value of a GMC is not the BIG TOY feature, but the community it connects you to. This is a good reason to also get signed in to the original forum as well. Yes, that format is old and clunky, but the multi-decade owners are over there and most love to provide knowledge and insight.
You can also look in on the groups over at Facebook, but do not take any advice from there until you have fully verified it. There are some that write there that have provided not just wrong, but dangerous advice.
Travel in health
Matt
