With you educated on all the options presented thus far, I do support your most recent experiment idea.
You can drill and tap the cast shell, but it can be unforgiving at times. I would suggest instead that you tape up the spring as you desire, and then solder SW-22 Super Flex wire to the rivet. The other end of the wire should be terminated with a ring terminal and routed to one of the conveniently already tapped 6-32 x 5/16 screws holding the front bracket to the shell.
While scouring the service documents, I did find this note that shows that your 1120-6 trailing truck may actually be factory as the later 1951-52 models had these upgrades applied from the earlier 1120 release.