I have a couple of thoughts on that subject...
Hooklift flatbed bodies are cumbersome for use as flatbed car haulers. They are terribly limited in recovery situations due to the inability of the body to support much in the way of winching other than very straight forward loading. You also have the issue of loose hydraulic lines (or electrical wires if it's for very limited use).
The standard deuce is too short for a "useable" slideback wrecker. You could build one or retrofit/modify a commercial one to be stout and all, but it's going to be very limited as to what you can load up on it.
An M36 would be an excellent starting point to set up a commercial light duty sub frame, a 19 or 21 foot deck (the "previous" and "current" standard in the light towing industry) would fit nicely on top of that.
Flatbeds in general are poorly suited to many off-road recoveries from get-go. Even very capable ones. They are large and difficult to "allign" to a disabled vehicle resulting in the temptation to abuse or misuse them, tree damage to the victim on the way out, the inevitable self recovery that could have been avoided with a much less capable but more maneuverable "conventional" type wrecker.
You'll also want to be very weary of the truck's frame. They are not nearly as stout as the "current" generation of commercial units, and older trucks of more comperable frame strength did have problems with the vehicle's frame, usually attributable to the operator although not requiring a really bad operator, and not requiring them to do anything too "dramatic" and still limited to on-road use. It will do fine IF you're very aware of what you're working with and use it accordingly.
I really, really want to do this, but I can't afford the flatbed parts, I can't afford a second truck, I can't see trading mine for one of a more suitable length, and I can't figure out what I would actually use it for, and it'd do nothing but get me into trouble in places that I really shouldn't be anyhow... And honestly the fact that I work on flatbeds is the only reason that this is appealing to me. Realistically, if I wanted to offer assistance for fun or for profit to people who were stuck, broken, mangled, or otherwise disabled off of the highway, I'd be looking at a conventional type wrecker. Even if I had a trailer to move the disabled vehicle once it's out of wherever it was. Much, much more practical, versatile, efficient, and sturdier. (although you'd still have to watch the frame a bit, just not so much).
But I'd still really like to do the flatbed...
Drag them out, and load them up on a trailer.
