So what's a good length to run with a 75HP motor?

+4"

Does the same apply to the front a arms?

no because that transfers weight forward, which in turn tranfers more force down on the rear whels.

think of it list this: Put your arm flat on a table (with your elbow down). try and raise your hand with your elbow still on the table. Now add weight/pressure to your hand and pick your arm up again (pivoting on your elbow that is still on the table). You'll feel more pressure on your elbow this time. 

i think i may have messed up explaining that but maybe you'll get the jist lol

How do you plan on making the stock aluminum swinger longer, stronger, and still lighter than the original design

PCS I'm planning on using motoman's engineering skills and programs to find out where I can lose material and not lose strength. A lot like what Brian said, is a no link rear suspension better than the full floater cushion lever design?

Again... application applies here.
  I've been told this by VERY experienced hillshooter/dragracer/fabricacator whom I buy products from..1" out front =+2" out back/swingarm.   In my experience I think it holds true.  I have multiple different setups.
..If you want to extend the stock swingarm the best/strongest/lightest way is to put 2 stockers together,.cut at a 45 degree angle..1 to maximize length..1 to maximize short.. +6" is possible...I recommend +4" max with the stock linkage/shock setup.  (Metaltech Fab does this well) The original material is very light and very strong.  This is a good way to go , I still have a +4" setup I have had for 6 yrs/ridden hard and going strong. I've had/seen others with foreign material added for length,.... they will break....unless its heavily reinforced.
As far as shock setup....the original linkage works good up to +4" if you respring and revalve the stock shock accordingly.  (HPR Racing).   If you go longer... the swingarm will begin to bow at the cushion lever piviot point and eventually break.   It's just too much leverage up front. (Well that's also if you ride Hard LOL)

If your only running flat sand drags a no link setup is easy enough to use.. basically a semi-ridgid suspension.

 That's why all the WUSSIE "fast" drag racers out west like to race at Dumont so they don't have to run through the Glamis whoops and include suspension and rider ability in the equation....

I ran a no link setup with a +9 swingarm back in '10 on my hillshooter/drag bike.This is a more demanding situation where you actually need the bike to go through the whoops at high speed(working suspension)....After respring and 4 attempts at revalving/riding/testing/revalving and so on...I had an acceptable ride but not great.
 I now have Much more appreciation for the Factory setups and see why they use linkage type geometry on the rear.   My current setups are modeled after the Raptor 700...structurally its a great fit but does require detailed fabrication. I have a +4",  +6",  +9",  +10" modified this way. I am EXTREMELY happy with handling and confident there will be no breakage issues.

The stocker is already very light.  Don't do any trimming..I would recommend adding a 1/4" plate top and bottom set into the "valley " so to speak for the full length of the arm, that brings it up even with the side rails..hardly even noticeable.   It will still be lighter than the aftermarket Chromoly arms and the front section is very strong.  Remember to make the cuts at the 45 degree angle for strength and more weld area.