These are pseudo coilovers that use a threaded sleeve to provide height adjustment to imitate the threaded body of real coil over suspension. I’ve used similar in the past as supplied by Ground Control, they are not the best solution by far but they do provide height adjustment if that’s all you are after.

The Bilstein front conversion is a little more involved than the rear and requires some welding skills. The front Bilstein HD strut inserts and the front sleeves are available from many sources including Paragon, Pelican, 944online, etc. Here is a German video with subtitles showing the procedure for fitting the Bilstein inserts into late 944 struts, you will also be fitting the threaded sleeves for height adjustment, the sleeves come with instructions.

Removing the torsion bars is relatively easy, plenty of procedures online, search is your friend. I dropped the carrier to remove them but it’s not required, I wanted to properly clean and power coat it, fit Elephant Racing spring plate bushes and remove (grind off) the “banana” shaped arms from the carrier as they are not required with coilovers fitted. However you can just remove the bars and retain the rubber spring plate bushes, especially seeing as the rear Bilsteins you are using also still retain the rubber bushes instead of the spherical bearings usually found on coilovers.

I assume this is for street use and you are not planning to track the car. I was running 350 front and 550 rear springs on my road car fwiw.