Yes you can reuse, I have done by extending them rolling a thin piece of aluminium in a number of grooves to open them up, as said length in haynes.
I have been binning stock tubes there seems to be no demand.
For me the advanages of the telescopic is that when assembling it's a fiddle getting all the push rod tube aligned and in place while bolting the head down but more important is changing tappets as said.
I have changed tappets twice with my telescopic tubes.
You can remove standard tubes later if you need to change a tappet but it's going to be a right pain becuse you can't saw them or you will get swaf inside, crush and bend out I have read but never done it.
I have used the Scat big mouth.
P1020199 by
Paul_Barr, on Flickr
P1040708 by
Paul_Barr, on Flickr