I am restoring a 67 S-code FB that I bought without engine or tranny. I have a 390 to put in it, but am still looking for a 4-speed. The veh serial # is 7FO2S142148, with 3:00 rearend and trans code 5. According to a toploader website, the 390 4-speed cars of that year all came with 28 spline output shaft RUG-M transmissions. The slip yoke on my driveshaft is, however, 31 spline. Perhaps at some time someone had put a 428 or 427 in the car. Who knows. The yoke can be replaced easily enough with a 28 spline, but my real question for all of you smart guys out there, is will ANY 4-speed toploader (RUG-N, RUG-E, etc) with the correct mustang tailshaft work? I have passed up some decent buys on "small block" toploaders because someone told me that I needed the "big block" toploader (RUG-M or equivalent I think, but I don't think he meant the 427/428 compatible large input/31 spline output). What is the real story here? I could use some expertise here. Remember, that this engine will be rebuilt very close to stock, with only a slight step up in cam, punched .040, and with the original 67 C70F-9510 Holley carb. I am located in central Indiana. Thanks in advance for any help!