We often have to drill out the pin, using a smaller bit than the 1/4" pin. Once the pin is "relieved" by this, you can easily punch out the remaining piece. Keeping the bit straight is important. As JWL said, make sure the shaft has no burrs (which would damage the new oil seals).
You will be removing both shafts, which releases the two 3/8" steel balls in the detent mechanism (located in the center of the housing). After removing the old oil seals and pressing in the new ones, you need to reassemble the detent assy. Put one shifter shaft in place, then replace the detent components (spring, plunger pin, and one steel ball). Now place the other shaft into its bore but don't push it all the way in place. Use a thin putty knife to hold the remaining 3/8" ball against the spring. Push the second shaft all the way in place....the cam detents will push the putty knife which depresses the ball. Once in place, just pull the knife out.
I suggest replacing the lever pins with new pins. We use a steel pin that we trim to be about 3/16" outside of the hole thru the lever. Use an anvil and your big hammer to tap the pin until you've peened it over nicely. This also "swells" the pin inside the lever and shaft so you don't have any slop when the lever is rotated for shifting. Repeat this for the other shaft and its pin.