Ah!
Sounds good. You know now that you'll want to get an accurate assessment of the trailer's weight. I like your plan...
Better than taking a leaf out (but not cheaper) is buying a slightly softer-rate spring. Chances are that they'd be a little longer, too, so some welding may be required to move eyelets and/or a shackle mounts.
You'll win in the end, though, with much more suspension compliance offroad and a nicer ride quality on the highway.
Here's a handy chart to help you find the right spring rate. It also contains some spring dimensions to help plan the modification:
http://www.stengelbros.com/UtilitySprings.htm
That link was brought to us by T&TTT User
bluzharp in this thread:
Spring Capacity By Dimension
Please pardon the slight deviation of the thread's course... 