GM doesent make 3.55 for the 8.5's- they do make: 2.41, 2.56, 2.73, 2.92, 3.08, 3.23, 3.42, 3.73 and 4.10
The second gen rear is longer, but can be cut down. I have used the carriers out of Trans Am's and put them in the Nova rear. I don't know , but I think GM did use a 3.90 in some three speed cars in a 8.5 housing. I know summit and Richmond gear offer them. I put one in a ChevyII two years ago. It was a second gen 8.5 rear cut down , with custom axels , and Ford big bearing axel ends. GM might not offer the gears, but you can obtain 3.90 and 3.55 for a 8.5 housing.