I have one out of my demon with the same combo but it's 49 3/8" center to center.I'm assuming you are allowing 1" play when the trans yoke is pushed in all the way?Mine are the small u joints also.
I have 49 1/2 in it now. I was going by a measurement post on here that said factory is 48 7/8. Mine is too long i believe. Its a homemade driveshaft and is off balance. Dont think its worth getting balanced being that much too long