How about some reference pics, for those of us not familiar with the PR-10 and its original equipment.
Are downtube shifters an option? A used Shimano 105 6-speed DT shifter set shouldn't be too difficult or expensive to acquire.
I'm no expert on index shifters, but the original Simplex RD was not designed for index shifting, so it's a bit of a leap of faith on your part to expect it to work, unless you happen to know that this functionality has already been established by someone else.