Lots of variables are at play here. My situation is different but some of this might be useful info. I have a 1963 C10 with 115” wheelbase. I found a salvage yard drive shaft that was the correct length and had the correct slip yoke. All I needed to do was swap out the rear universal joint for a conversion u-joint and it bolted right up. Drives great and without vibration. My 1965 C10 also has the 115” wheelbase but I had a driveshaft shop swap modify my original driveshaft. They swapped out my original slip yoke for a correct slip yoke from an S10 truck. That required a conversion u-joint for the slip yoke. The rear u-joint did not need to change. They shortened the driveshaft about 3”. Dennys Driveshafts has a website and a free worksheet that can help you measure the correct length driveshaft for your application.
The pictures below show the salvage yard Blazer driveshaft I used in my 1963. Neither of my trucks have an adapter plate for the T5. The conversion u-joint pictured was correct for my application but might not be correct for yours. The Blazer slip yoke pictured is correct for a T5.
