My Eagle's trans has been rebuilt with about 15k miles on the rebuild, and shifts around 3300rpm with my foot in it. During real gentle starts, it will shift at 1500rpm. I will manually shift it if I need more power like getting on the interstate and such, and will shift a little over 4000rpm. Our 258 straight sixes though don't really liked to be revved past 4500rpm I've found. Even with performance upgrades, they will start to valve float above 4500rpm. They are low revving engines almost like diesels and make tons of torque down low from right off of idle to 2800rpm.
As far as the slippery feeling shifts, your second gear band may need adjusted in the trans. It explains how to do that adjustment in the service manual. Mine has nice firms shifts between gears even when in the throttle, so I wouldn't do too much hard shifting until you check the adjustment on the second gear band. That's actually what broke in mine several years ago, when I had to have it rebuilt. I lost second gear completely. I would start out, and when it shifted into second, the engine would just rev all the way up and then go into drive.