Converting a 3 speed manual to 4

I am interested in doing the same thing. I have a 73 Dart with a 318. Will the driveshaft be the same length, or will it possibly need to be modified?
Are there any real advantages to that extra gear, or is it just a matter of personal preference? I have an 8 3/4 rear end with 3.23 sue grip. Right now I am running about 2500 RPM at 65 mph when I am on the freeway. It would seem that extra gear would relieve a bit of stress from the motor, but I may be off base.

Thanks in Advance,

Jason