They may be able to shorten the strut to the right height then cut a new thread inside the end. If not just as you say, cut section from the centre then weld it back together. The sleeves are a snug fit so the outside of the strut needs to be flush.
I had lowered King springs at 220lbs, these were fine to use with regular KYB (GR-2 i thing they are) shocks and provided a sporty/firm but compliant ride. Those inserts are for a standard length strut I got KYB AGX's (for a Z31) from Datsport, arrived in 2days , they are 4way externally adjustable, right length for sectioned struts and cheaper than Konis. You will need the strut top hat locating hole drilled out a bit bigger to suit these. Make sure you have new bump stops fitted too.
I currently have the otomoto c/o kit with 6gk/mm (340lbs) springs, KYB AGX's in the front, they are stiff on the street but it handles like a go kart , I would recommend 6kg for track work only. I'd get the 4kg springs all round.