And, you still may be able. We know you have the C-4 but what rear end do you have? The 6-3/4" or the 8"? The 6-3/4" would have bolts holding on the back cover plate. The 8" does not. Its cover is welded on and the gears only accessible by removing the whole center section. Knowing the rearend you have will help others to know if they have a shaft that will fit. You can also measure the center to center of the front and rear U-Joints and how short the shaft you have is to come up with a proper total.
Remember that a rear end in full droop will result in the shaft sliding rearward from the trans. So, you can't measure with the car jacked up. The general rule I've heard is that with the car at full suspension rest the driveshaft should have about one inch forward travel. Thus, if you slide the yoke into the trans all the way, then slide it out 1" and measure from the front U-Joint centerline to the rear yoke centerline you should get the needed length.
If it is any encouragement I have a '64 Studebaker that I swapped in a GM 700R4 trans. While it took sliding under a number of cars (oh, the thrill of the hunt) a '78 Buick 4 door Skylark provided the exact sized, bolt-in driveshaft that I needed for under $20 at Pick Your Part. So, maybe there is a C-4 splined yoke/driveshaft out there of the proper length and U-joint size.
I an curious. You had a C-3 Pinto driveshaft and it was to small/large with the C-4??? You acquired another Pinto driveshaft that is too small. The Pinto wheelbase never changed, they used the C-3, C-4 and the 4 speed manual and the 6-3/4" and 8" rearends. Did they have six (3X2=6) different driveshaft lengths???