Softails's come from the factory with the lightest setting for the rear shocks (for <180lbs solo rider). However, they can handle well over 400lbs with no problems with a simple preload adjustment.

The service manual is a bit vague on how to set the preload. The procedure I use is fairly simple. Its based on the procedure from fatbillscustoms.com, but their website is gone now. It was fairly detained. I've made some tweaks to it for simplicity... Enjoy...

1) Jack the bike up so the rear tire is off the ground. The more clearance you have the easier it is. Don't forget to strap the bike down. You don't want it falling on you while your wrenching underneath it.
2) Loosen the jam nut all the way out. Its a 1-11/16" nut. If it doesn't want to loosen easily (which is often the case), soak it in some penetrating lube.
3) With a spanner wrench (HD p/n 94448-82B) turn the adjuster plate counter-clockwise (so the shock housing moves towards the front of the bike) until its up against the jam nut.
4) Then take a light colored paint pen, or a piece of duck tape, and mark the 6 o'clock position on the adjuster plate to use as a reference point when doing your adjustment. Don't worry if they're not in exactly the same position on both shocks. Thats normal.

Now your ready to adjust the shocks to your weight/load.

5) With the spanner wrench, turn the adjuster plate clockwise no more then 4 complete turns by counting how many times the mark from step 4 crosses the 6 o'clock position. 0 turns is the stiffest setting, and 4 turns is the softest setting.
6) While holding the adjuster plate in place with the spanner wrench, tighten the jam nut up against the adjuster plate by turning the nut clockwise.
6) Now repeat the exact same process on the other shock. The key is to adjust both shocks equally.

The general rule of thumb I use is:
4 turns for load <= 180 lbs
3 turns for 180lbs < load <= 235lbs
2 turns for 235lbs < load <= 300lbs
1 turn for 300lbs < load <= 375lbs
0 turns for 375 < load

Examples:
235lbs solo rider: 3 full turns
235lbs solo rider + 32lbs pack: 2.5 turns
235lbs rider + 150lbs passenger + 32lbs pack: 0.5 turns

I like a slightly stiffer ride when running solo (3 turns for my 235lbs fat booty), but run a softer ride when my girlies on the back (1 turn). Adjust your shock to taste by 1/4 turn increments till you get it dialed in for you and your riding style.

Note: you may be tempted to skip step 1 and not jack the bike up. Resist the urge. A stock ride height with the bike on its kick-stand, you may be able to use a crowsfoot or similar to get the jam nut loose, but you will have little to no room to use the spanner wrench. So take the time to do it right and jack the bike up. It only takes a minute (including strapping the bike down).