New member on the hunt for the right coach

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hammermonkey

New member
Jun 27, 2026
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Hey all, I'm a carpenter by trade and lover of classic vehicles. The kind before computer assisted designs and federal regulations made everything on the road look identical. Naturally I love the look of the GMC and I'm hoping to find a coach to outfit with a custom interior plan.

I'm leaning towards the 23' footer for easier maneuvering and parking.

If anyone has interior dimensions of the 23' from end of cockpit to the rear wall that would be really helpful for my planning phase. From eyeballing a few pics it looks like there's about 15' to work with?

I'd also love to know about anyone with experience in upgrading their 455 and the th425 to improve torque and hp significantly. Looking for well trod path to get there.
 
Welcome to the forum, hammermonkey. I'm sure a GMC motorhome will scratch that itch.
There were not many 23' GMC's built. Most built were 26', but there are some 23 footers out there.
There are some on this forum who have swapped out their 455 with a Cadillac 500. Some have converted to diesel. I think there might be one person who converted to an 8.1L gas engine. With the exception of the Cadillac 500, there is quite a bit of work to do to convert to a different power plant. Not impossible, but not easy, from what I have read.
Perhaps there are some on this forum who can chime in on how they added power to their 455's. I know many have moved from the Quadrajet carburetor to fuel injection.
I would encourage you to check out https://gmcmi.com as well. If you do go the GMC way, you will definitely want to join GMC Motorhomes International.

Best wishes!
 
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Hey RvRev2, thanks for the welcome.

I've been perusing gmcmi a bit and looked at those schematics but couldn't pull a clear number for the dimensions in question.

Found a few of those swaps you mentioned, but I'm more of the mind of sticking with stock components rebuilt for more power. Even saw there's an aftermarket olds big block now, just unsure of the packaging challenges. Might tug that thread if it truly bolts in.

I can handle all the interior work myself so I'm looking to throw some bucks at turning a coach into a "muscle" motorhome.
 
Christo said:
Drawings at https://bdub.net/GMCdrawings/

What is your goal for improving torque/HP? A Cadillac 500 as mentioned in a 23' with 3.70 gears and limited slip diff will scoot.
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Oh those are perfect drawings! Thanks!

My goal is just to have fun driving it and not have to worry about merging in East Coast traffic where a turn signal indicates to other drivers "please accelerate as fast as possible and ensure I can't change lanes even though I'm running out of road"
 
hammermonkey said:
I've been perusing gmcmi a bit and looked at those schematics but couldn't pull a clear number for the dimensions in question.
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GMCMI & bdub's site might not have the same resolution scans, I do not know.

Here's the enlarged scan from bdub's:

And my screenshot of the PDF:
IMG_8018.webp

It shows 176.13" from the step back to the back of the last vertical aluminum rib.

You'll find that GMC standard dimensions aren't so standard. Things tend to sag and move around after 50 years. If you try to design a GMC in CAD without field measuring the actual coach you'll probably be doing a fair bit of revising.
 
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Thanks Richard that's good to know. I'll incorporate some flexibility into my plan so I can adapt to real world conditions. I'm very accustomed to plans not matching reality after years of working with architects!
 
Depending on where you live, there are clubs that would welcome you to a gathering where you could see a few coaches in person. When I did an engine rebuild, went back almost 100% stock except a Melling "RV" cam and the matching springs. My coach already had the 3.42 differential - only available from Cinnabar. Jim K out west has 3.55 and 3.72
 
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You can build a 455 or a 403 with heavy duty components, balanced and blueprinted, about 8.5:1 compression with a Jim Bounds "Kryptonite" cam. Also, a heavy duty transmission rebuild with the "blue racing" clutches, Kolene steels, kevlar bands, and a HD torque converter. Install a BIG transmission cooler, and go with a 3.70 or 3.55 final drive. All will help.
 
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tyler said:
Depending on where you live, there are clubs that would welcome you to a gathering where you could see a few coaches in person. When I did an engine rebuild, went back almost 100% stock except a Melling "RV" cam and the matching springs. My coach already had the 3.42 differential - only available from Cinnabar. Jim K out west has 3.55 and 3.72
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That's great advice, thanks. Is there a resource that lists clubs by location?

Bhart said:
Put a turbo charger on the 455. Ask Jim K. at GMCRVPARTS how he did his. 800-752-7502
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Haha that's quite a feat given the packaging. I'll wait until I take possession of a coach and have done more independent research before blowing up their business phone with a bunch of questions.
 
hammermonkey said:
I'll wait until I take possession of a coach and have done more independent research before blowing up their business phone with a bunch of questions.
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The sooner you start out tapping into the GMC community knowledge base the better. Jim's a walking GMC encyclopedia, has as much experience with them as anyone on the planet and actually enjoys talking on the phone. He's not the type that pushes people to buy stuff or expects a phone call to turn into an order.

I've said it before, people come to the GMC for the hardware and they stick around because of the people.
 
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