I put dot 5 in my 67 disk/drum GTA in 1982 and there were zero issues (I took the system apart for a full vehicle restoration in 2010.) At the time (1982) I put on a rebuilt MC, new rear brake cylinders, and rebuilt the K/H front calipers. Did not touch the lines, distribution block, or proportioning valve other than drain and flush them in place. Never noticed a problem with brake performance. The brakes were actually much improved but since the components were rebuilt/replaced at the same time I can't say if the dot 5 on its own was better or worse for performance. Over that nearly 30 years there were some years that I did not drive it at all, but I would start it up and back up and down the driveway. I think I went about 10 years between bleeding. For all of these reasons, with no negative experience, dot 5 went back in as part of the restoration.
67 289 GTA Dec 20 1966 San Jose