Yup, as others have said you'll need to get a custom TH400/4L80E trans adapter "Kit" which includes: Block Adaptor Plate, SFI Flywheel Assembly, Crankshaft Adaptor and All Fasteners and Dowel Pins. If you go with Donato kit you'll need to tell him to include the spacer, he didn't make one for mine and it left a large gap so we had to make a custom Flywheel to Converter Spacer.
I wanted OD so went with the 4L80E which is 80% TH400, just with an OD. It's f#%ken FUN!!!!! I thrash the shit out of the Batram DAILY and it's held up real well and chirps the gears through all 4, hard, 1-2nd (and sometimes 2-3rd) shifts send the arse end out hard often haha. You'll also need pedals and from memory you don't need a new pedal box as the pedal boxes are the same, I left my manual pedal box in and just changed the pedal rods over (left the gas pedal and e-brake).
It's not that hard a conversion, just depends on how far you want to take it. Transmission, pedals, shifter set up, firewall mount dipstick, trans cooler, driveshaft, trans-mount (you can adapt the factory one easily), time & money. How expensive depends on your truck, if you want wild, get custom, if you want simple, stay basic. I added a Compushift Transmission Controller from Twistmachine.com, Hurst Promatic-2 Automatic 4-Speed Truck Shifter, custom console set up, Mercedes AMG Paddle Switches, etc but then I like to be different and have OCD lol.
I love mine, so much more of an all round truck, quicker, consistent and now I get that cruise factor too. As for wiring, for mine, there was really no major wiring other than the Compushift. Being a manual to start with means there is little in electronics, it's a basic manual trans, going from 48RE to TH400 would involve more wiring work. In short, remove the old T56, no wiring, slam in the auto, make a console and then go have fun lol. Video below of the Compushift, and a link to my conversion page. The video is more about the gear changes, the "jolts" between it are from the 4L80E, even mild driving it's FIRM but I can change how hard it shifts using the Compushift. If you have anything specific in mind, just ask and I'll try my best to help.
www.carlwalski.com/Batram2.html
Cheers,
Carlwalski
THE BATRAM
[ame=http://www.youtube.com/watch?v=wJzqIvBijBA]YouTube - Compushift Display Demonstration[/ame]