drive shaft help

[1979blkchevy]

Hello back with a drive shaft problem i did some research and this is what i came up with my truck has a 6 inch lift on it and i had one heck of a noise coming from the front end so i took the front drive shaft out and everything was shot and the driveshaft seem to be alittle short in length!! so i was talking to some guys and they said i could buy one and have it stretched to fit or i could buy one that fits it they were saying i needed about a 18inch to a 20inch driveshaft! how do i measure a driveshaft so i know for sure on how long of a shaft i need and what are the pros and cons on streching a drive shaft thanks again for listening