hi mate
yes you can use the 5 speed box from the sierra (me and my brother done a MK2 escort
conversion a few years great fun) but you also need the engine mounts from a RS2000 plus the radiator form the RS2000 a single piece prop shaft as if you use the split type it keeps coming lose due to the extra power going to the back wheels (unless using a RS2000 rear axel) we used a axel from a escort 1600 sport much quicker off the mark and now the hard work starts as you need to cut back the transmission tunnel by 6" as the 5 speed box is longer than a standard escort box also you need to either cut out the under side of the tunnel to make room for the box to fit (again due to the size of the box and the bell housing be much wider then weld in a plate to cover the hole up or if you dont fancy that get a lump hammer and knock the under side and keep trying the box to see if it fits) thats about it mate but if your stuck with any thing just ask and i will help you as much as i can
good luck get some pics up when your finished