I'm going to put a manuel valvebody in a th400 anyway, if I can do it to a 4L80 too I might go that route for the overdrive.
Well hell yeah. You've got nothing to lose there. Cost a few more bucks for the trans, have to cut down the driveshaft and move the crossmember back a tad. Past that, not much different.
I wanted to do it so I can run a real low rear gear and still drive on the freeway.
One thing to make sure of, if you're not running the OE column shifter, and I wouldn't, be sure to get one of the models that support the 4th Gear. A ratchet shifter is a real good idea for a manual valve body. The B&M Quicksilver and the Mega Shifter both support 4 speed autos and backwards compatible to 3 speed auto's too. Also offer reverse valve body pattern if you ever go NHRA Certified. Also available in a taller truck base. The Megashifter doesn't claim to be compatible to 4L80-E but I'd be about certain it's cuz of the pan mounting bracket for the cable and you can get one or mod one. The QuickSilver does claim to be 4L80-E compatible. and is also my choice since it's $30 less, has a taller stick than the Mega Shifter, and I've also had Quicksilvers in 2 of my street/strip vehicles and love the way they function. I've NEVER had a miss shift or an accident hitting N or R even using the standard forward OE valve body pattern.