Here's the lengths for the shocks you are interested in: raptor 660 (15.5"), yfz450 (~16.25"), yfz450x(~17"), or yfz450r(~18.25"). There are several things to consider when changing the shocks out for longer ones. Ideally, the extended and compressed lengths need to be correct in order to utilize the full travel of the shock and still have a safe amount of travel left in the balljoints. There are three setups I would recommend for you to look into.
First, would be the setup I am running on my bike: A diamond j kit at +2 setting, with raptor 660 front shocks. This setup will have close to the correct extended/compressed lengths, and ride considerably better than stock. This is also the cheapest option. (I have less than $100 in this setup, but you have to wait or the deals). The issue with the diamond j kit is that suspension geometry is effected negatively. Bump steer becomes a problem, and unless you are like 150lbs or less, the suspension will be too soft. (Luckily I'm light, so this setup works for me). Check my build thread for more information on this setup.
Second, would be aftermarket warrior a arms. With Fullflight +2 or +3 a arms, I believe you can run raptor 660 front shocks. Yfz450, yfz450x, or yfz450r shocks will fit with the Diamond J relocation kit. This kit however puts the shocks at very upright angle, effecting some of the shocks performance. Also, the extended and compressed lengths in this setup are way off from my observations. Geessfore runs a set up like this, you can look at his build thread for some pics of it.
The third setup would be 400ex long travel a arms. Reference Duneracerweston's "how to" thread for more information on this, but for this set up you must be willing to be +4 inches wide and throw done some serious cash. If I remember correctly, this set up will run the yfz450r front shock. Another similar option would be long travel 300ex a arms. These would mount the same as the 400ex arms, but only be +2 or +3, and most likely use the yfz450x front shock. However, this is also an expensive option.
One thing I would recommend you stay away from is running the Diamond J shock conversion kit with stock a arms. The shocks will be completely up right, and you will have all kinds of issues with extended and compressed lengths. Hopefully this answered all your questions, pm me if you have anything else you have questions on. Also, If anyone notices something I missed, please correct me.