You have an [unknown] ute, & you want to make the rear springs stiffer?

Generally speaking, the rear springs in any commercial model are stiff enough, but probably not low enough. To get it down, some use lowering blocks, but i personally like to have the springs reset by a reputable spring works. This is a much better solution than trying to find another rear spring to fit.

I presume that you mean the CA18DE engine. This is a good choice, but to do it well, you will need to know your way around all the tricky bits that surround the fuel system, including the tank mounted fuel pump, & have the ability to work through the relatively complex issues of the electrical sustem & the computer.

It really is a fair bit of work involving chassis work, major fuel system changes, electrical work, body work, & some mechanical work.

Alternately, you will need the money to pay someone to help you through the tricky bits. I believe that some trans tunnel mods will likely be required, & when it's all done, you will need to get it engineered before it can be legally registered.
This is not really a job to be done in the back yard by an amateur & does not really come under the headings of quick, easy, or cheap.

If this is your first car, or first special car, then it might be a good idea to work yourself up to it by repairing & maintaining the ute first, then perhaps rebuilding the original engine, or upgrading to something more exciting within the same engine family. You will be learning all the time.

The CA conversion can come when you are in a position of being confident that you can finish it. In other words, just be carefull that you only bite off as much as you can actually chew.

The CA18DE is a good engine choice though