Find another 1985 S10 with an automatic and swap all the needed parts over. About the only way to make sure to get everything you'll need. You'd be surprised how many little parts are different from one to the other.
Or simply drive the automatic and save yourself a lot of work.
This actually doesn't seem as difficult as swapping the auto for a stick. I'd think that since you've already got a hole in the floor for the shifter you could just use an after market floor shifter. The 700r4 that came stock should fit without issue and bolt right up to the drive shaft. And you could just remove the clutch pedal from the assembly instead of replacing the whole pedal box. What am I missing here?