According to this post:
http://www.pro-touring.com/threads/9...-vs-sbc-weight
which got the info from GM, the weights are:
19244097
LS3 = 415lbs (LS1 = 409lbs)
19201330
ZZ4 = 511lbs
http://www.chevroletperformance.com/...inePreview.pdf
So if you are looking at motors alone, and they are placed in the same position, you'll change only 100#. If you changed transmissions as well, you'll have to figure that into the equation.
If your car is an "average" 3500#, 55% on the front is 1925#. So 100# is only 5% of the total front weight. Probably not enough to require a spring change (depends on rate, but you're likely ok).
The easiest thing to do is get the swap done then drive the car and let your butt dyno tell you what it thinks. That's the nice thing about true coil overs. . .you have a ton of spring rate choices, and you can change them at any time for not much coin!