I prefer a bit more magnification than most folks, and I stretch my 45-70 a bit farther. My wife has a 4.5-14x40mm Nikon on her 1895GS - even with the short barrel and lackluster velocities, it's a 300yrd deer killer. A guy doesn't NEED 14x to shoot 300yrds, but it's sure nice. The 4.5x bottom end has never been a problem, even for ~20yrd shooting.

That 3-9x should treat you just fine. The "Ultimate Slam" scopes aren't Leupold's high end lines, but they're serviceable. Considering $350, there are a lot of good quality scopes in that range, so you might look around a bit at other options and see if anything tickles your fancy, but I wouldn't say there's anything WRONG with the Leup Ultimate Slam.