Joined: Jan 2010
Posts: 9,815
Likes: 1,790
From: Northern California
Bikes: Cheltenham-Pedersen racer, Boulder F/S Paris-Roubaix, Varsity racer, '52 Christophe, '62 Continental, '92 Merckx, '75 Limongi, '76 Presto, '72 Gitane SC, '71 Schwinn SS, etc.
I expect indexing trouble when using the 6 and even 7-speed SIS derailers with an 8s cassette.
Usually there is a significant problem with the indexing accuracy when the two largest cogs are selected, as the derailer does not then move quite far enough, and noise results.
I've had this problem with 105, 600 and Dura-Ace SIS derailers btw, but can't recall having this problem with your older ~MT62 Deore derailer, so it might work ok.