I'm pretty sure it will. If you have a measurement of a 400 dipstick, I can compare it to one of my 4L80e dipsticks.
Thanx GSX man. I can actually pull out the HT400 I have from under a pole of auto-dookie and put them side to side. I will report the info back. I might just take abunch of photos and do a little article on the BBB-4L80e install. Thanx buddy.