I just bought a 89 LeSabre w/109,000 miles car is in pretty good shape, it has the
standard speedo cluster - I have a cluster I pulled from another LeSabre in the junkyard
that has tach and gauges - does anyone know if this swap is feasable? Does the cluster get speed and tach, etc. info from the ECM? Hopeful thinking that I can just change the oil pressure and temp sending units and the rest will work?

Not sure on this specific vehicle, but usually they require re-pinning, if not replacing the entire plug. You will need to find out which plug does what on the old cluster versus new one. Best bet would be if someone has FSM'* that could help us.

I'm currently working on this project and also an in-depth HOWTO manual for those willing to take the plunge.
I CAN, however, tell you from what I have found so far.. if you are going from an 89-91 "granny style" cluster to the "newer" style cluster, you need to get the wire harness that fits the newer style cluster. The older ones have 2 wire plugs that are set into the dash, so when the instrument cluster is placed in the slot, the plugs automatically plug into the back of the cluster. The newer style looks like it plugs in.
Also, the older style have big pins that wrap around the plug, and the newer style has pin holes because the pins are inside the cluster.
So.. I'd found out how to take about 30 screws out and remove the dash. I've found that I'll need a different plug, AND I'll need to re-pin everything (which I have mapped out).
I've also found out that the "Information Center" is a different module that sits to the right of the new cluster. (has the SES, Cruise, Security, Low fuel lights), which also needs to be re-pinned but not really necessary for driving.
It'* going to be a hard project, no lie there, but at the same time it'* going to be fun.
Will post more when I find out more.