727 trans

In the early '60's some trucks, vans, and "fleet" vehicles had small block 727's installed with a special adapter. About 1967 or 68 Mopar came out with the slant six 727, which was a direct bolt up. As stated a 904 will do the job for most slant six situations. As for value, just like any thing else, it depends on how bad someone wants it. They aren't rare, but they are not always easy to find.
PS: I'm pretty sure I have one or two, in my stash.
PPS: Now if you have one of those slant six, to small block 727 adapters, those are rare, and people are always looking for them