Going to 36h will gain you roughly 10% more strength, but it isn't cheap requiring new hubs and rims both.

I suggest you buy only new rims and have a good wheelbuilder build you a proper wheel on your existing hub. When I build HD wheels where durability is a prime concern I use 2 gauges of butted spokes. Usually it'll be 14/16g on the right flange, and 14/17g on the left. For really heavy riders, I've used 13/14g single butted spokes right and 14/16g left. Using different gauges left and right provides more equal stress in the spokes on a dished wheel, and butted spokes tend to be more fatigue resistant improving wheel life.

All in all, I say a better build will probably do you more good, than simply adding 4 more spokes so ask around and find the guy with a reputation for durable wheels, tell him what you're looking for, and take his advice along with having him do the work.