Guys, this project never got done, but back on my mind now. Does anybody know what those cross pieces on LJ tops consist of?? I tried some heavy magnets, and no dice, so I assume it is not metal. If they are steel strips I will try to hit them plus a few more bolts, but not looking like it.
I did this on my LJ then went to CO and back from the east coast carrying a Thule 700 pod and a cargo basket on Thule square bars & Tracker II towers attached to the long (68") XJ tracks. The pod was longer than the roof - with at least 200lbs or more on the racks - there was a good bit of heavy off-road banging when fully loaded which eventually led to a failure. The good news is that the weak point was the XJ tracks and I believe that that is preventable - which I'll address later.
Here is what I did ...
First, note that the hardtop is made up of several pieces that are bonded together. Look on the inside of the top. Along the left and the right edges of the top-horizontal section, there is a seam where the left/right vertical piece joins the top-horizontal piece. The side piece comes up and bends in with about 3/4"-1" of flat, horizontal section that is bonded to the top-horizontal piece. (The top-horizontal piece continues on the outside, curves down, and is then bonded to the side piece again, forming a longitudinal box channel - pretty strong) That horizontal, flat-bonded section on the side piece is where I located the tracks. That way they are right along the box channel, on a double thickness, and the fasteners actually hold the seam/bond together. It also give you almost the exact same spacing as on an XJ if you are using the factory cross-bars.
I used 1/4" (by 1" IIRC) Stainless, Hex-Socket-Head Cap screws with Nylock nuts inside. I backed the fasteners on the inside of the top with a full length of 3/4" x 3/16" steel.
I cut the steel to length then taped it the XJ track to mark the hole positions and then drilled the 1/4" clearance holes in the steel. The tricky part was that the steel had to be relieved where the 2 lateral cross pieces overlap the mounting area (to about 1/16" thickness - thinner stock would have to be fully cut out around it). Fortunately I have access to a milling machine so it was not hard - it doesn't need to be too precise so it could be done carefully
with a grinder.
I then taped/clamped the steel to the inside of the roof and drilled up using the steel backer as a template. Notes: Take the top off to do this. The top is a fiber reinforced structure wear the proper safety equipment. Use a GOOD drill bit and go easy on the pressure to minimize breakout damage as you go through. I did not try it, but a brad-point bit might be a good choice to help minimize breakout - experimentation required.
The failure mode that I had was that the heads of the mounting screws pulled through the bottom of the XJ tracks. The problem is that the access holes on the top of the tracks for the mounting screws are not big enough for (significantly) larger screw heads or washers to be inserted to better spread the load. As I see it, two possibilities exist to get around this. I need to either fish washers into place at each hole from the ends (probably using glue or caulk to hold them in place for assembly) or I will fabricate a full length backer strip using 1/8" aluminum and insert it into the track. I will probably do the backer strip since it offers more strength IMO.
I've seen no evidence of damage to the top itself, despite the banging it took with the free floating track at the back - enough banging to crush the XJ track down about an 1/8" where the cross-bar foot was mounted.