Rip it out and stuff it in. The 700R4 is a tad longer than the TH350/PowerGlide you have now. You will need all of the wiring and computer out of the van to install in your Impala. The pan might be a bit tight with your cross-member as truck pans are lower slung since they don't have road clearance issues that cars do.
The wonder of the Chevrolet is the amount of parts interchangeability. a set of headers to fit your car are the same if you put a 1955 265 small block in or if you put in a small block Chevy Vortec engine out of a 2001 pick-up truck (even though the "D" port heads have an issue with exhaust leaks the headers will still holt right up and seal initially).