Stick with a stock swinger or an aftermarket one.  Large amounts of stress on the pivot, linkage and frame once you start going longer.  Don't get me wrong, you can jump a little.  I will get a little air here and there with my duner(+6).  Nothing huge though. 

The +12 on my 1100 is there to keep the nose down.  Which it has.  I can launch with it at @ 8,000 and it will just carry the nose  perfectly.  Most of the time it just feels light.  Not even coming off the ground.  As I get even more comfortable with the bike and setup, I can only imagine the front end being carried about 6" off the ground.

Just understand that a +12 is a huge arm.  Might be overkill for a 500 dragbike regardless.