Update:
We decided to mount the front turnbuckles to the front of the jack brackets, using a home-made metal plate system similar to Bigfoot and Bad Habit's. Although it would have been nice to get an attachment point further back on the camper to get a decent amount of forward pull, it seemed like all the options for doing that would risk wrecking something that would be a pain to fix. We made the plate out of 1" angle aluminum, with holes drilled to attach it to all 3 bolts holding the jack bracket to the camper. The way we oriented the pieces, the tunbuckles exert just a bit of forward pull - enough, it seems, to keep the camper held snug up against the front of the bed.
Initially I was hesitant to go with aluminum, but it's easy to work with and if it starts to show signs of wear I can just get my neighbor who's a metal worker to recreate the same thing in steel. Thus far, after the maiden voyage, things seem to be holding up with no signs of damage to the aluminum.
The TorkLift AnchorGuard turnbuckles showed up in good shape ($127 well spent!). We trimmed down the threads to allow for the short distance between mounting points.
We've decided to stick with the rear bumper mounts for now, since the rear AnchorGuards are damped (the fronts are spring-loaded) and the bumper seems pretty solid.
I picked up an 8' x 4' piece of 1" thick Foamular ($20) to slide between the camper and the rubber mat on the truck bed. Turned out to be the perfect thickness to get the camper sides up off the rails. Haven't fastened it to the camper yet, eventually I plan to glue it down and maybe put a few screws in with fender washers to keep it in place. I can also cut some extra pieces to cover the bare wood on the sides of the bottom in back, which are exposed to water coming up off wet roads. The Foamular is tongue-in-groove, which will make it easy to get everything sealed up and weatherproofed.
We took the rig camping over the weekend, and It seems to stay put well, even on washboard dirt roads with occasional big bumps and a short steep rocky hill. As Jim (JaSAn) mentioned, the friction between camper/Foamular/rubber mat/spray-on bed liner seems sufficient to hold everything in place nicely with no shifting. When we took the camper off the truck and looked at the Foamular, it was indented by the ridges in the rubber mat, which probably resulted in loss of height of about 1/8 inch (the depth of the ridges), but the sides of the camper are still clearing the back of the bed rails comfortably. It'll be interesting to see if small changes in position of the camper each time we take it off and put it back on the truck will result in further compaction. If so, we may need to start over with a new mat without ridges (like a horse stall mat), or maybe just keep our current setup and attach a thin layer of rigid material over the bottom of the Foamular (this looks like it would work well:
http://www.homedepot.com/p/Unbranded-Hardboard-Tempered-Panel-Common-3-16-in-x-4-ft-x-8-ft-Actual-0-155-in-x-47-7-in-x-95-7-in-832780/202404545).
Overall, this is HUGE improvement over the crappy chain/ aluminum turnbuckle setup we started out with, in both function and appearance. A much cheaper fix than I'd expected, with minimal modification of the HappiJack system Hallmark installed.
Barring a change in the way Hallmark installs their attachment points on their campers I think this is probably the best way to mount a Guanella to a short bed Dodge. I'd love to use a frame-mounted system, but the front frame mount (the one that really matters) would put the front turnbuckle at an angle that would pull the camper backward. We may end up going with rear frame mounts instead of the bumper mounts, but at this point it seems like that would add expense and weight to our setup, possibly needlessly. Time and off-road miles will tell...
Photos of the (nearly) finished product below.