You can convert the rear axle by swapping out the chunk to a 55 and up truck unit but I believe you will need to swap the transmission out for a open drive unit.I think the 4 speeds from those years have a replaceable tailshaft you can convert them with but not the three speeds.I could be wrong but I am sure others will respond.
actually using the 216 in a speedster project. I was gonna use the 3 speed and rear in it also if I could shorten the torque tube. I'm going 235 t5 in the truck and repurposing the old 216 3speed.
I don't see why not on shorting the TT. The inner shaft is a tube not solid,twisted one up in a knot behind a 261 one time. A lathe would be handy but could be done without one. A careful straight cut on the outer and line it up good should be OK. The inner treat like a regular D/S cutting the tube at the end pieces.