I not sure for your 6lp impeller, but for my 6lya I use this tool - JS
-09-47163-01 -Johnson impeller puller. Basically screws onto the threaded end of the impeller, then a bolt in the center is tightened to extract the impeller. I added a longer bolt to mine. Makes extraction a piece of cake.
For replacement I just get the impeller seated on the splines as far as possible by hand then put impeller tool back on and tap the impeller into place, remove the tool, put on a new o-ring, cover on and done.