http://www.camaros.org/driveshaft.shtml#usage
Driveshafts for 1967 and 1968
In 1967 and 1968, Camaros with the 6-cylinder engine or the small block engine used a driveshaft with a length of 49.96" and the smaller 27-spline yoke output shaft with a seal diameter of 1.502 inches. 67-68 Camaros with the 396 engine received different engine mounting brackets. Due to these frame mounts, big block engines are offset approximately 1 inch toward the passenger side of the engine and 0.5 inches forward. Because of this, big block cars with manual transmissions have a driveshaft that is 0.5 inch longer (50.46 inches).
Most transmissions have an overall length of approximately 27.75 inches. (For manual transmissions, this includes 6.375 inches for the bellhousing and 21.25 for the transmission itself.) The TH400 is about 0.5 inch longer, or 28.25 inches. Because of this, the driveshaft length for the big block and TH400 combination is the same length as the L6 and SBC engines. But the TH400 uses a larger 32 spline output shaft and requires the larger yoke with a 1.875 inch outside diameter.
Driveshafts for 1969
All 1969 Camaro driveshafts are 49.56 inches in length. In 1969, big block engines were still offset toward the passenger side, but the "front to back" engine mounting location was now only 1/4 inch forward compared to the the L6 and SBC engines.
The yoke design for all applications was strengthened in 1969 by adding metal to the internally splined neck. The yoke was also lengthened, which allowed it to be used with not only all the L6 and small block applications, but also the big block manual transmission application.
The TH400 yoke design was also strengthened, but it still had the larger trunions and u-joints. Hence a unique driveshaft was still required for the TH400 applications. The document below indicates that the TH400 yoke was also going to be used on 396 and 427 with manual transmission applications, but this was not implemented until 1970 on the M22 and 1971 on other Muncie models.
There was also a third driveshaft used in 69 for all TH350 applications. The length and the yoke are the same as the 3950194 driveshaft. It is unknown why this driveshaft was required.