Mine was done already i bought the car this way.
It has caltracs on it with a mono leaf and i was going to put a 5 leaf back on to get a better ride, i took everything off but with the caltrac removed It's like the box is too wide?
There are some washers on it already but without the caltrac it's going to be even wider. i might leave the caltracs off.
The spring should slide nicely even a little snug into the spring box. That's the way I did mine. Just make sure you measure the hell out of everything before you cut. I've got pictures on my thread somewhere.