Intake manifold renovation
You betcha this is an important part for your GMC. The 455 & 403 intake manifolds are having the problem of cracks forming between the secondary ports, when this crack reaches the floor of the interior exhaust gasses from the cross over port contaminate the vacuum and performance then goes out the window. If you have low vacuum, rough idle, poor performance under load or intermittent performance issues, this may be your trouble. In the past, the fix here was either to “block off” the crossover with stainless steel plates or change out the intake to an aluminum one. The new intake was always my choice but you would need to build up the floor when using the aluminum intake on a 455. The 403 can use the aluminum intake without raising the floor. Check your intake by unbolting the carb. and inspecting the thin wall between the 2 large secondary ports (look at the first pic here). If you see a hairline crack where this one is, you may want to investigate further.
Here is what you do: have the intake removed from the motor, either box it up and send it to us or find a local shop to clean and magnaflux to see if there are any other issues with the intake itself. 
Reinstall the intake using a high performance fiber gasket set, you will find that lost power you may have never had before. If your vacuum issue was from the crack in the intake, it will be gone. I betcha 75% of the coaches out there have a crack of some sort in this area. The crack may not be effecting your performance YET but then again it may. 
You will need to retrofit the carb. to use an electric choke (which we do automatically) since you will be doing away with the hot gasses that ran the old heat riser choke system. This is one good reason to send us your carb at the same time and have that done. Read more about our carb. remanufacturing below. Reconditioning your intake will usually run @ $200 plus shipping if we do it for you, thats less than buying a new intake and you won’t have to build a “doghouse” to hold the higher aluminum intake option. Give us a call if you need more input on this needed service. We have done many of these in house and now feel it important enough to offer the service through the mail.
Carburetor remanufacture
“Rebuilding” a carburetor is a function of taking it apart, cleaning it well and reassembling it with new gaskets and some internal parts. Thats all well and good but with the age and use we deal with on GMC carbs., a simple “rebuild” is not enough. Butterfly shafts are worn out, meter rod hole are out of tolerance, plates are warped and a pile of other problems keep your original carb. from doing its proper job. We offer a remanufacture service on your original quadrajet carburetor. Send us your nasty old carburetor, it will be disassembled, parts checked and reassembled, checked on a flow bench then set up on a big block motor and gas tested bringing it back up to proper spec and operation plus adding some esoteric setting for the carburetor to run better in your GMC. 
Not only will it work like a dream, just look at it, man is that nice or what! Hey, and put it up on your refinished intake and 
you’ve got a piece of artwork! This service will usually run $285 but may be a bit more if major parts need to be replaced. If you want it to work right (guaranteed), send it to us.
We have so many more parts “just for your GMC” there’s simply no room to list them here. Call with what you are doing with your coach and let us help you locate the pieces/parts you need. We not only have parts but install adjust and maintain coaches so he offer you more than a part. Let us help you get your coach is top notch condition. Give me a call at the shop