700r4 what is the right Tail housing to use.

[Clavo106]

have look on threads but it got really confusing with all the lingo.. just need a straight answer if possible.. I have a two chassis one th350 and another th400 ..im wanting to replace the th400 with a 700r4. I have two 700r4 with different tail housing one I believe ins a 86 out of a pickup 2wd with mechanical speedo.its a parts only trans./ the other is completel form a 89 or 90s pickup 2wd.. what tail housing works and can I swap elec speedo for the mechanical one thanks.