Much has been said about Kelly trying to switch the defense from the 4-3 to an attacking style 3-4, and from reading many of the comments many people have little understanding of what it'd really going to take to make the switch so I figured I'd put my 2cents worth in with this fanpost.
I'm going to go from a position-by-position look and the needs of the front 7 since when it comes to the Safety's and Corners not much (if anything at all) changes which is good. So this means that this post will be on the front 7 and what is needed from them.
Defensive Line: DE/NT/DE
Right Defensive End- This Defensive end needs to look to be disruptive and get pressure on the QB, when you're looking for a guy like this you're thinking somebody like Justin Smith who can be extremely disruptive while still being able to get to the Quarterback. While playing DE in a 3-4 you also need a guy here who can take on 2 or more blockers (Example of this would be a TE/OT with a RB chipping) in order to eat up blockers.
Nose Tackle- By far the most important position in the front 7 of the 3-4, if this guy isn't able to draw constant double teams to keep the MIKE and the BUCK clean then you're going to be in for a very very long day. You basically want a guy who's sole purpose in life is to be a little asshole and forces the C and a OG to block him every play while still being able to collapse the pocket. An example of a current player would be Shaun Cody for the Texans, he does a good job of keeping the LB's and amid all of the LB injuries the Texans had he constantly gave the ILB's an opportunity to make plays.
Left Defensive End- This End you're going to want to be more like a DT than an End, somebody who like the NT will eat up multiple blockers but you need them to be capable of winning one on one matchups against RT and get in the QB's face. Clearly JJ Watt is the best in the NFL at this however you don't see a 3-4 DE do that ever, Watt is a rarity, not the norm. Think about Brent Keisel with the Steelers, he's been good at this for years.
(NOTE: The LDE and the RDE's responsibility's can switch depending upon where you're main pass rushing OLB lines up)
Right Outside Linebacker- This guy is most likely going to be your main pass rusher, you won't see him drop into coverage to many times and he needs to be able to play the run well. Look at guys like Aldon Smith and Clay Matthews, they're great pass rushers and are capable of playing the run.
MIKE Inside Linebacker- This is basically a guy you want to be running your Defense, he's going to be the downhill run stopper who adds just enough against the pass to be effective. The MIKE has to be able to work his way through traffic and fight through blocks to make tackles. Essentially this is the MIKE from a 4-3.
BUCK Inside Linebacker- This is a very important piece of the 3-4, probably only 2nd behind the NT. This is a guy you like to be more athletic than the MIKE, he's the guy you look to do just about everything from coverage to providing an inside pass rush. The best representation of this is Bowman from the 9ers, he's the best I've seen at this.
Left Outside Linebacker- The best way to describe this guy is a hybrid between a WILL LB and a DE. You want a guy who can occasionally get after the QB and a guy who plays extremely well in space and sets the edge against the run. Ahmad Brooks has to be the only guy I've seen be able to do this without making it look hard.
wild_eagles 3-4 Line-up:
LDE- Fletcher Cox, because he's quick, disruptive and can eat up blockers while still collapsing the pocket. I feel he has the athleticism to become a very good 3-4 DE.
NT- Antonio Dixon is really the only guy I see with the size to play here, however I feel that Cox could also be in the middle and keep the LB's clean while providing the inside pass rush necessary. I think as of right now a NT isn't on the roster.
RDE- Derek Landri and Cullen Jenkins are a tie right now, both are athletic enough to play DE in a 3-4 and both are very disruptive. I'd also say you can fit Cole here due to his super athletic ability and the fact he wins double teams all the time while grading out very well against the run.
ROLB- Brandon Graham is the ideal fit for me outside of Cole, however I feel when making the switch you need to roll with the young guy here so he can grow, Graham was very disruptive as a pass rusher in 2012 and was probably one of the few bright spots on this team. He has good bend, stays low and I feel standing him up could make him much better.
MIKE- Demeco Ryans, I don't feel I need to explain this but I'll state that he played in a 3-4 at Alabama and in Houston. He played well in each and keeping his role more like the 4-3 MLB would be better for him and the team.
BUCK- Jamar Chaney is my fit here, he is an athletic compliment to Ryans and he plays well enough in coverage to be a threat, I'd also say that his experience as a MIKE would be greatly valued because he's able to play the run and pass. (My second option would be Kendricks)
LOLB- Kendricks thrives in space, he played OLB his last year in college and provided some good pass rush there, this season he wasn't asked to rush the passer very often yet I feel he has the ability to develop as a pass rusher while still provide the coverage we need in space.