If you are doing a c4 conversation on a flatty a 1970 Mustang drive shaft is a direct fit

 

I'm impressed that you found a '70 Mustang driveshaft!

 

The other piece of information necessary for this is to know what rear end you're using, and which pinion yoke, if you're using a 9". There's a lot of variables with different parts that may make your find correct or incorrect for the next guy.