I used the Progressive Suspension shock, already mentioned, on my rtw trip last summer, and it served me well, being a major improvement over the stock unit. Progressive's fork springs might also help your fork seal problems, although I don't really know that, just surmising. But they do result in better handling on a loaded bike than the stock springs. However, I was not carrying anything like the load you are planning (I was loaded to only about 290 lb on my trip, including my own weight).
I, too, would have concerns loading a KLR that heavily. According to my owners manual, the maximum load capacity of a KLR 650 is 401 lb., including the rider, so you are almost 100 lb over that. A subframe mounting bolt upgrade would definitely be recommended. There are two types available, one being simply four higher grade bolts and sold by a variety of companies such as Happy Trails, and the other utilizing a single, very strong upper mounting bolt going completely across the subrame and requiring drilling out the bolt holes in the subframe slightly (and two stronger lower mounting bolts). The latter is sold by Dual Star, http://www.dual-star.com/index2/Bran...pgrade_kit.htm
With a few modifications the KLR 650 makes a great adventure travel bike, IMO, but I don't believe they are the best bike for carrying heavy loads. I know of two being ridden solo but with very heavy loads and over very rough terrain in South America, and both developed cracking of the welds along the main frame tube underneath the tank, and have heard of one that was being ridden two up in the US that also developed a cracked frame (no other details available).
Hope this is of some help.