Re: 'Do i cut the pvc to the same length?'
No.
The OEM preload is something like 30mm. I don't know why they do that. Filling fork tube with a steel/plastic spacer seems rather counterproductive to the whole idea of 'suspension' to me.
Preload preferences vary by rider. 8mm is a reasonable place to start.
Short version: Assemble the cartridge; spring, clip (on bottom of spring), and fork cap-threaded all the way down on the rod..NOT into the tube. Leave out the cups (originally on both ends of the OEM spacer).
With a spring from Fredette (probably 470mm or so long) you wil have space between the top of the spring and the bottom of the cap. Measure that space. For sake of argument let's say you measure 13mm.
The cups that you didn't put on the cartridge rod will take up
about 3mm of that space (measure the flange thickness that will be taking up some of that space to be sure of the measurement).
That leaves you with 10mm of space. Cut your spacer to that
plus whatever you want for preload.
In our 'argument' case, that would be a spacer cut to 18mm.
Cut the PVC square. A miter box (powered or not) is helpful. Square whatever wonky leftover you have with a file..deburr the edges.
Put it all together.
On the last step of reassembly, hold the cap, turn the tube to thread the two together. If you turn the cap..the fork will make a 'scritchy' noise on compression.
Yes. That is a technical term.
Fork spring preload is an important part of fork setup. Saying 'Forget about it..sounds like too much trouble' is a mistake.
There is info to be found elsewhere on this board..and maybe you already found it. The above is short and close enough that it will work for 'ya.
Good luck.