OP, I suggest you grab that Hyperglide cassette I linked from Ebay. The tooth counts aren't posted, but it looks to me like 13-14-15-17-20-24-28-32. If I were you, I would use your original 13, 15, 17 and the HG 20, 24, 28 sprockets to get a nice progression. If your RD can manage the chain wrap, another nice one would be 13-14-16-19-24-32.