“Pull in your gut”, hey… it’s not your fault! The front body clip of your GMC is spreading out. Go to the front of your coach and sight down the side. Most of you will see a bulge over the front wheel fender well. 
You will see that the plastic front clip is pooching out. This is caused by the plastic body structure that was bonded together with 40 YEAR OLD GLUE let go and the body is pulling away from the base structure of the body. From cutting away the body to swedge in the Workhorse Duramax/Allison front wheel drive train, I know about front body bonding failure. 
Yep, that’s how I put those Duramax coaches together! Drove the Workhorse chassis right up under the body then stuffed it up in with the lift…. but that’s not what we’re talking about… lets get back to how to fix the body bulge up front….
On late model coaches (1976.7.8), there is an inner support structure welded to the body floor. You can drill a 3/16″ hole in the aluminum body strip 
then “bevel the hole opening to allow the screw to “countersink below the surface of the aluminum 
Check out the screws GM put in…. pitiful… 
This is the first screw Ray put in after putting in 6 screws to pull the body in spreading the movement out 
So pulling in the body along the body mold you end up doing this 
Compare before and after…
On early coach bodies (1973-4-5) You will need tp remove the inner side panel and install a bracket to screw to… There is no inner support (you have 3 support rods)
So look closely to the side of Cal’s coach over the front wheel well .. can ya see it… you will when the color is on 
Thanks for stopping by, search for other information on the web site for exterior refinishing. .
Just tried the search function, works pretty good! Lot’s of great info on here…
so did you use self tapping screws or drill and tap the holes ?