Thanks to all who offered suggestions.
I was a bit timid with the first truck. I even removed the side frames and then, of course, had to put the springs back in at the end. I vowed not to remove the side frames and create the re-springing problem on the second truck.
Here's what I did. I cut the wheels off the axles. Cutting the axle once near one of the wheels was sufficient to remove both wheels. I did the same to the other axle. This gave me a clear shot at two of the rivets. I drilled them out with my cordless drill. Drilling out the other two rivets at this time would have endangered the side frames. Using my Dremel with a cutoff disk, I cut away the remainder of what I considered non-essential material. I cut across the top of the bolster where the pin comes up. I continually evaluated what I needed to leave. It turned out that the bolster itself with no attachments is sufficient. This left me with an open shot at the remaining two rivets. I drilled them out and now I had the bolster with both side frames attached. No need to remove them and no springs to re-insert. I vacuumed and brushed the truck. Blew out any dust in the journals and then added a drop of LaBelle oil to each journal and then loosened the screw holding one of the side frames just a bit and inserted the Intermountain wheel sets. I re-tightened the screws holding the side frames.
I attached the trucks to the car and checked the height of the bottom of the floor with my KD gauge and determined that one or two shims will be needed to bring the couplers to the correct height.
Hope this helps the next person faced with this challenge.
Ed