the 6 speed commonly used (well when people put a 6 speeder into these anyway) is from an focus st170. but then youd want a diff as well which is an extra £5/600 before any fab work is done to make it fit. which is about the same as a rebuild

my advise would be to go for the cts box

does the rs1600i have a casing like that to hold the ARB then?