I don't think Johnson seems like a big enough guy to play a 3-4 DT slot, and honestly, even the best 3-4 DT's don't get after the quarterback much. That's not what you want from a DT in that scheme...in that scheme, the DT's job is to "Not be moved" so that the defense can't run right up the middle. Guys like Vince Wilfork in that scheme get 2-3 sacks per season and still make the pro bowl, because "getting after the QB" isn't the job of a 3-4 DT.
This is why I don't think that the 3-4 is a good strategy...it's so hard to find one of those really good, run-stopping DT's. You literally can't use a 4-3 DT in that scheme, you get eaten alive on the ground. I don't know who else is available but if you want to make that switch...Raji is a free agent this offseason.
Johnson seems like a good pick for the Bears but as a 4-3 DT actually.