How much lift with stock magnum roller lifters?

Yes you can, a fellow I know has done it. In fact I have a few set up that way in my garage. The internal parts combo came out of disassembled 350 chevy lifters. He has 5 seasons on his Dart using the converted lifters, he run his stuff hard and has had no issues.

Was he able to get all the slop out? It really wouldn't matter I wouldn't think as long as you included said slop when you adjusted the valves. Just make sure you were pressing down on the pushrod side to take the slop out.