It will not fit. Generally 2wd and 4wd transmissions are not interchangeable.
Also, although the 400 trans is strong, the lack of overdrive and the lack of a lock up converter means it's a high-revving gas sucker.
Finally, the 400 trans is 27 years old had may have been sitting around for a while and may not work anyways. Even if ti worked fine when it cam out does not mean it will work fine now. I have had used transmissions sit around for a while; maybe the seals dry up or something, but sometimes they do not work right without a rebuild.
IMHO sell the 400 trans and put the dough towards a rebuild of the 4L60.I am not a big fan of installing used auto transmissions because it's way too much work to swap them to find out if they function properly.