You will have to look for a set of cases, cant use just lower, you need both the upper and lower. Watch ebay, the 77 and 78 f is harder to find they are much rarer then the K. You may be able to use 77 and 78 K cases but I cant say for 100%. The biggest thing with the f motors is the output shaft and sprocket stick out farther from the block for the wider stock rear tire on these models. The cases are also wider at this point to support the longer output shaft. You would be best to stay with the 77 and 78 f block so you know it will fit. I do have a set but not really looking to sell. If you cant find a set hit me up later and we can talk about them, cant guarantee I will sell though.