I don't think the bed would be too narrow for a standard truck camper, most are pretty narrow in the center to clear the truck fenders. Go to
www.gmupfitter.com then publications then light duty then the year and download the c/k series and compare the truck bed and avalanche bed, I think width they are pretty close.
I'd look for a deal like
http://columbus.craigslist.org/rvs/975044902.html
You might have to modify the front corners to clear and maybe eve shorten it. But most truck campers are wood frame with aluminum or fiberglass panels nailed/screwed/stapeled to it so shortening it wouldn't be too hard. Just remove the rear corner trim carefully then trim the outer skin then cut the frame then reattach the rear and seal and retrim everything.
Slide outs are not hard, basically build a drawer with heavy duty drawer rails.
Ok, looks like the narrowest point of the bed is 47.25", slightly narrower than a full size truck but wider than a small truck like a colorado or s10 or dakota or ranger. I'd probably start there, look for ones that were made to fit the smaller trucks and measure the width of the center. The main issue will be length, you will have to shorten it due to where the rear axle located in relation to the bed (look at the upfitter guide, its near the front). You will want to keep all the weight forward if you can.
One suggestion to start now is get the battery tray for the diesel trucks so your "house" battery can be under the hood. That moves it out from the camper area. Put any charge controllers/isolators/inverters up there, there is plenty of room, I've set my truck up that way.
There is a large empty hole under the cab behind the transfer case where I'm going to put a water tank then cover the whole bottom with a skid plate.
If your close by OH it looks like a fun project if you would want some help.
I see your looking at DIY, I've thought the same too. Looking at wood working forums I see that a certain type of cedar wood is water resistant which I thought of using so if I don't get an area sealed well I won't have to worry about frame rot. I've seen fiberglass 4x8 panels at hardware stores but they have been textured so I was thinking of asking about ordering plain flat ones. They are usually used for shower stalls or public restrooms.