The Jeep adaptor plate is what you need...as others pointed out.
Go Here:
http://www.carburetion.com/adaptors.asp
and you can learn all about adaptor plates, etc. What you want for the SuperSix is the Small Two Barrel adaptor plate--measure it to be certain (don't take my word for it).
The actual plate is here:
http://www.carburetion.com/Weber/webern ... =99004.551
$30 and its yours. When you get the thing in hand find a flat surface and lay down some 120 grit or so sandpaper and give it a good "honing." These adaptors can be pretty rough at times. Dry fit it on your rig, figure out how you want things to lay out (i.e. the PCV port, etc). Once you got it figured out use some Permatex #2 (of equivalent) and put it together with the supplied gaskets. Do NOT use any goo on the carb bottom.
The point about the 32/36 v.s. the 38/38 is a good one. There is a 34/34 that is out there as well. If I had nothing but money I wouldn't mind trying them all out to see what they are like--I think most people call this fully tunable fuel injection! I like the idea of the dual 32/36 manifold--that would be fun and it would look pretty trick. I wonder how it would work on cold mornings though.
Got to love the internet--you can get smart on adaptor plates and other arcane stuff in a click of a mouse button.
Anyway, don't give up, the Weber will work. The linkage might take some tweaking--I needed a cotter pin secured pivot pin with a big flat head on one side for my Weber's linkage...I sawed off a ten penny nail and drilled a hole in the nail for the cotter pin, problem solved.
Otherwise the Carter BBD that SlantSixDan has will work right out of the box--just remember that the reason the BBD is replaced often is because it is a flogged >25 year old carb--apparently Dan's are new (??!!) so they should work just fine.
One last note--the Weber's SOUND great, even in a truck with a leggy A-833 O/D. I have some looonnnggg gears so it can be a little boggy if I don't stay in the torque curve, but if I drive properly that Weber just howls. I love it!
Sidebar:
I need to deep six my fully functional, but skinny exhaust system to see what that does. After the 2-1/4" exhaust pipes go in I suppose I am in for a head shave--I wonder if the larger valve game will do anything performance wise, but it can't hurt I suppose. Anyway, my Slant Six is a stock motor (sans the LeanBurn stuff) and the Weber made quite a difference. Now, I have no idea how it ran when new with the one bbl and a fully functioning Lean Burn System so what I deem an improvement might just be "stock" performance! I am pretty certain that the Weber is a little congested with the skinny exhaust system though, so keep that in mind for later improvements...