MSD ignition installed, now I'm thinking transmission time?

So I went to JEGS and bought the MSD 6AL box/coil kit, with BOSCH platinum plugs and new silicone wires, etc. I can honestly say, this is the most striking change I have ever seen in the performance of a slant six! (have had 3). It idles smooth, warms up quick, has even and reliable acceleration, and even has a louder and deepr exhaust note! I have had no cold-morning issues this week (40 degrees?), and it warms up riiiiight to its running temp and stays there. WOW. Anyone wanting a quick way to smooth out your motor's running attributes, THIS is the way! Oh, and $300. Installed. NICE!

Another thing is, I have been leaning toward ridding myself of the stock 904 and moving up to a later transmission with OD from a later slant six car/van. What are my options, as far as trans swaps that will bolt-up to a slant six? I really need better freeway economy (of course carb/headers will help too, that'll happen by June). I'd like to know if there's an AOD that will fit my slant without too much trouble. None of that tailshaft-bolt-on crap for $2700. That's twice what I paid for the car :) Any ideas?

-Hunter