Excerpts from old but relevant boilerplate -  designed to cure you of
GMC lust:
===============================
You say you're bitten by the GMC bug, and realize the financial
and time commitments. Whether that's true or not you are wise to
ask someone who has outgrown the bug!

You'll NEED, IMO, really good shocks, rear stabilizer bars, and
Wallace front hubs. I consider the latter essential, having suffered
several failures in the original design.  You may well WANT a number
of other mods from Caspro, Cinnabar or others. Personally I would go
for the final drive mods to get closer to the right ratio of around
3.70:1.

The GMC's cult following is well-heeled enough that SOMEBODY will have
or make the parts.  Caspro has a lot of suspension & drive train parts
including a 6-speed AT!  Engine & transaxle are Olds Toronado.
Recently I have been unable to contact Wallace Bearing,  the supplier
of the crucial after-market front hubs, and believe the original
Wallace died and his son has gone outa business. I am confident that
GMC motorhome club people could put you in touch with someone who can
solve that problem.

There have been cases of fires due to failure of the oil cooler
hoses, which DEFINITELY should be replaced with the braided style.

Stick with the 455 engine but do not expect more than 10 mpg unless
you are a real featherfoot. DO NOT tow with the dang thing. Gear
ratios are wrong.  You will find some people who do tow with them
but I think they are nutz.  I was really unhappy with handling when
towing.

There are several negative aspects to this lovely, superbly designed
coach.  Virtually ALL of them are chassis related, since the coach
body is arguably the finest stuff ever mass marketed. Any old crock
of this vintage will need seals, hoses and most rubber replaced.
Windshields are notorious for fogging at the corners but I think can
still be bought for money.  You may have to replace the entire air
suspension system, tubing, valves and bags.

But no amount of money and maintenance will solve the following
problems, which I believe to be important flaws:

    FWD sucks on a heavy MH. You will be limited to good roads,
    gentle grades, and good traction conditions.

    The rear suspension affords the best ride of ANY small MH
    but has lousy braking.  Nothing wrong with the brakes    
    themselves - but the front tandem is on a leading arm, the
    rear on a trailing arm, so braking force is impossible to
    balance.  Result: long stopping distance and smoking rear
    tires.

    The final drive, regardless of gears, is too fragile,
    especially if the engine output is enhanced at all. Mine
    broke the case trying to accelerate from a light towing a
    Pontiac Fiero.    My friend's broke driving solo. Understand
    that EVERYTHING in the drive train except the hubs is    
    straight off the Toronado line. And the hubs are different
    only in having a special 8-bolt mounting flange for truck
    wheels. Its a tribute to the Olds design that it holds up
    at all in this service.

    The steering in inherently sloppy, with two U-joints and
    two sliding joints between the wheel and a steering box
    designed for pickup trucks. (Latter is bad news/good news -
    bad news because it is too light for the job, good news
    because every junkyard will have one.)

There are many people (most moderately wealthy) who have come to
grips with the GMC's weaknesses and are very happy with them.  We
found it entirely impractical despite the admitted charm of a coach
that was stylish, far better built than anything else available (16
gauge aluminum skin, for heaven's sake!), and as smooth a boulevard
cruiser as ever came down the pike. We spent most of the time we
owned ours (A VERY pretty '73 Glacier model, a blue & white
23-footer) crawling around fixing stuff and modifying stuff and
spending money.

I trust that will give you another POV... if you decide to buy one
anyway, save this message for a year and let me know how close I was
to the truth!
====================

Will - The self-appointed Curmudgeon of Sill Hill

Cinnabar GMC refurbishes.

Will:

R:

I can't prove a negative, but I find no evidence that a Ford engine
will bolt up to a Toronado transaxle.  Bolt patterns aside, fitting a
chain drive to the V10 crankshaft is not something I'd try.  IOW, I'm
skeptical.

Will - The self-appointed Curmudgeon of Sill Hill