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. In an effort to "build a better mouse
trap" we have found that filling the cross over port with zinc alloy is
much more reliable.
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 and have
this done.
Here is what you do: have the intake removed from the motor,
box it up and send it to us (you may also want to ship us your carb., read more
about that below).
We will have your intake (not someone elses old one) glass beaded and
magnafluxed to be sure there are not other problems, then we will fill the cross
over with the molten zinc alloy and file the slag smooth. 
and ship it back to you. Reinstall the intake using a high
performance paper gasket set (Mr. Gasket #404 set works great). 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 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. Filling the crossover and
reconditioning you intake will usually run @ $210 plus shipping,
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.
Carb. remanufacture "Rebuilding" a carb. 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
carberator. Send us your nasty old carb., 
It will be disassembled, parts checked and reassembled then
set up on a big block motor and gas tested bringing it back up to proper spec
and operation. 
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 $235 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.