n20mstr has a 400 in an A-body. He would be the one to answer about the fit

While I haven't sat the two side by side, both the 400 and 727 would be required to have virtually the same main case length and dimensions, because both are 3 speeds and you need the room you need to get the parts in it. The 400 has a lot shorter output shaft and tail housing. I THINK it would be a no problem fit, but don't know that for sure