68 Calipers are like Corvette calipers, and many many thousands of them have been rebuilt with stainless steel sleeves to prevent the rusting issue in the bore that caused the originals to leak. I've had several sets rebuilt over the past 25+ years (by Lonestar) without any subsequent issues. When I did mine, Lonestar took my original calipers, rebuilt them and returned them to me (so I could keep the original dates/codes). Not sure if they still will do this?
I've had Whitepost do a couple of Master cylinders for me (for a '60 Corvette and for my '69 Z28). I 'wanted' stainless steel at the times, but White Post only does them with brass sleeves. They could not adequately explain to me that brass was better and if it is I do not know the answer, but I'm sure it (brass) is easier to work and machine than stainless (which I suspect is why they use brass). White post also recommended against use of silicone brake fluid, but again without adequate reasoning. I've had silicone brake fluid in a couple of cars for over 25 yrs without ANY subsequent issues or rebuild work necessary. I prefer it for cars that are seldom driven.