I'm not familiar with that specific kit, but what I have done in the past...
Figure out how much you need to lower it.
I would dry fit and hold the suspension to the chassis and fit the chassis into the body to get an idea of how much. Once you get an idea of how much you what to lower it, check measurements for the following process before making changes. Adjust total lowering by the smallest measurement allowed (front or more likely rear).
Front coil spring suspension
If there is enough material to work with I'd file down the springs, spindles, shocks. Then file an angle in the lower control arms and if necessary the frame rail so they will fit together.
Rear leaf spring suspension
If the springs have an arch, lay it on a flat surface with arch up. Measure from flat surface to arch. That will show you how much you'll have to adjust (total height minus amount to lower). I carefully held the springs above a candle (use caution not to burn your fingers or melt/catch fire to the springs), giving slight pressure to the springs to un-arch them. Once they get soft enough they will bent easily. Check measurement and repeat as necessary.
If the springs are flat to begin with. If there's not enough material to remove from the chassis where the springs mount to I make lowering blocks from the scrap sprue. Install between rearend housing and springs.
Don't forget to file down the rear shocks also.
Hope that helps a bit.