1st. The only problem with SS is that it has a potential to gall. But, you should be ok as is. Frankly, I wouldn't bother but... If you are worried about it you can look into an automotive grade bolt.
Second - you are pulling the shaft through both a stuffing box and cutlass bushing. if you are having trouble moving the shaft forward by hand, 1st position the coupling where you want it on the shaft and tighten it, then get a couple of longer bolts to put through the shaft coupling and into the transmission coupling. Tighten them evenly, a couple of turns on each side until you pull the shaft and shaft coupling up snug to the transmission coupling.
Also, I replaced the 4 nuts/bolts that hold the two couplings together. I used SS bolts and SS ny-lock nuts
