I can't believe a WP guy would tell you that. WTF??? The starting rate is way way too light anyway. Even if it did fit.
I would say stick to the straight rate spring. The linkage does the progression on the LC4. The LC4 sping is also 59mm ID. The normal PDS springs for other KTMs is 62mm ID. It doesn't sound like a lot but it really is when you put it on the shock. The 62mm ones are all over the res.. I think even with a centering ring of some sort on the shock colar it would rub like crazy.
The stock springs are appropriate for a 100lb dude. I don't know why KTM ships them this way and doesn't even offer springs in high enough rates.
For your weight I would say you need around an 8.4 in the back. But with the load an 8.6 would probalby be ideal. And a .52 for the front. I know this sounds stiff. But it will make your bike ride really nice and also not bottom every time you hit a dip or cross a wash.
If you spring the back you need to do the front. Your bike will ride horribly with only doing the back spring. It will make the head angle steeper and cause head shake at speed. Also as far as the weight being on the back wheel? The 640 A is really front biased. They dive like crazy under braking because if it. So at the very least you need the correct springs for your weight. If you really want to go for it you need to build a real midvalve in the fork. This is the best mod I have done to adventures to help control the dive on and offroad. You notice it the most when going down a steeep hill offroad. With stock valving the front tends to ride low. The bars want to turn to one side or another and it is hard to control. With the midvalve mods the front stays up and you can ride it more like a real dirtbike and charge down a steep hill.
Sorry for going on and on. Anyway I can get you springs if you need them. If you are out of the country it just takes a while and the shipping kind of sucks. But the rear springs I do for the Adventures are custom length and rates to fit the application. If you source the spring to a spring maker where you live let me know and I can give you some specs to get what you need.