After a few phone calls and considering the input here, I think that I've come up with a solution. I got some 1/2" aluminum rod and cut it to length (for the 3.9, they came out to 3 1/2 inches). Due to the shape and the fact that I have the full divider, one solid piece wouldn't drop in without huge gaps. So 10 rods are dropped into each of the corners, JB welded in, and the smaller cavities between then will be filled with either more JB Weld or block filler (probably the JB as it's a lot easier to find in Alaska).
I bought this Dakota for Point A-Point B transportation, but somewhere in the process of tearing it apart to get it running, it turned into a full-fledged build. Cowl induction, modded TB and manifold (BTW - great thread on modding a TB!), reworked heads and valvetrain including a 1.6 roller conversion, headers and dual exhaust dumped behind the cab, dual electric fan setup, and a few other little goodies. Bodywork was started this weekend - welding the tailgate, shaving the tail lights and emblems, roll pan with a surprise for lights, blow-through into the extended cab, too many interior mods to list, all topped off with a 2/4 drop.
All just to get rid of it in three months when I get stationed stateside. No wonder my wife thinks I'm nuts...