Yep, it started in 1970. Here is some info on the c4's:
Don
C4's were produced and used in production cars and trucks from 1964 to 1981, when they were replaced by the three speed automatic transmission with lockup torque converter known as the C5. In this timeframe there were several versions, each with significant differences that, depending on your application, might make one better for you.
From 1964 through 1969, the C4's made had a 24 spline input shaft and front hub assembly, and with the lower spline count came a slightly smaller diameter on the input shaft. This year range is still desirable, as its case is identical to the 70-81 case fill C4 transmissions, so the old 24 spine parts can be upgraded to 26 spline. However, the valve bodies in the transmissions are different. The valve bodies fall into 4 different groups. 64-66, 67-70, 70-81, and the Pinto valve body. Keep this in mind when selecting a C4, since the valve bodies cannot be interchanged between year ranges (a 64-66 valve body wouldn't work on a 70-81 C4).
Bell housings came in both 157 (small) and 164 tooth (large) bell housing sizes, with matching flex plate sizes.
In 1970, the C4 got an upgrade. The input shaft and front hub were strengthened by enlarging them to 26 spline, increasing the input shaft diameter, but also making its torque-converter and input shaft non-interchangeable with the older 24 spline 64-69 C4's. The 1970 through 1981 case fill C4's also came in 157 and 164 tooth bell housings, but a new variant was created, the pan-fill C4. Sporting a 164 tooth bell housing and flex-plate, and a differently designed bell housing and transmission case, it was put mainly into trucks during its usage. There was also an oddball C4 bell housing, the 143 tooth version that came in Mustang II's and Pintos, possibly Mavericks as well. This transmission is not desirable at all because of internal differences, like a weaker input shaft and oddball valve body. Also be careful, there are some 4 and 6 cylinder bell housings out there, the starter is at approximately 10 o'clock, looking at the trans from the back. Obviously this is not desirable either.
Now what is the difference between the pan fill and the case fill C4's you might ask? The difference is in the transmission case itself, and the bell housing. On a case fill C4, the bell housing meets the transmission case at nearly a 90-degree angle, with the case remaining flat as it approaches the bell housing. The case fill gets its name from the fact that its dipstick/fill tube goes into the transmission case. On the pan-fills, the dipstick/fill tube goes into the transmission pan, with a threaded fitting. The routing of the pan-fill dipstick tube sticks out from the side of the transmission much more than a case-fill, and in a place that can cause more of a problem with header clearance. The other large difference with the pan fill is that the transmission case flares out as it nears the bell housing. The angle between the bell housing and transmission case is much less than a case fill C4, maybe only 60 degrees or so. Also with the pan-fills, the bell housing bolts onto the case with bolts outside the front pump, where the case fill bell housings attach with the pump bolts.
