Longies in reverse

I frogged the Cargo longies down to the legs last night. I decided I'd try knitting down from the top, meeting the legs and Kitchnering the legs on. I sure hope that's going to work! I cast on 100 because I thought (from the gauge I was getting before I frogged), that that would be the right number for gauge, but it was still coming out too big (I'm aiming for 10" across) so I am trying it with 80 stitches. I also started on really small needles, but I'm ready to switch to the bigger ones now (4s I think) because my fingers are cramping up something fierce. I hope that works itself out soon!