Ok, I see your dilemma.
I have never used an extension shaft. The only thing similar I've done was repair broken armatures with tubes from a watch pin. Results were not perfect. I'd avoid this idea. However by all means experiment. Put your choice cluster aside. Find another to play with. eBay is a good source if you have some extra flow.
If it were me I'd get a hold of two schematics, showing pinouts for your cluster/harness, and schematic from a similar year which has a closer layout to what you want. (Camaro, or Truck). You may find the harness u have will support things like tranny temp, tach, etc. if you are making an entirely new appliqué, it doesnt much matter if it's a roadmaster cluster. A lot of Impala SS guys will use a Camaro cluster, and run my overlays. It's a direct fit. So, I'm thinking u have a good shot at making another cluster work. You can remove the circuit board from the face, and extend the wiring to the individual armatures. I have heard that many mid 90's Buick speedos go bad, and that may drive their prices higher in the used market.
Looking at your pictures a thought I had was leaving the speedo on the same plane. Use some plexi sheet, cut an opening for the entire speedometer. Mount the other 4 gauges to this panel. Screwing the armature housings to this plexi. You could mill out the back if it's two thick. I'd get a salvage truck unit and dissect it. This plexi idea is sort of inverse of the old design. The speedo is set back, and the 4 small gauges are in the top plane.
I created overlay kits for Camaros which avoid calibration issues, but the 4 smaller gauges can be calibrated using resistors or volts. The speedo is tru
Is trial and error even on a test bench; required any time you pull the needle. If you use a 2 good truck (or other) ones. You can at with one format, mock up, then get serious about the final one. May not have to remove its pointer. Many do NOT return to zero on there own.
Shoot me a pic of your plug, and jack on the back of cluster. Rmacewen1@yahoo.com