8 3/4" swap info

Hi all, I decided to rebuild, and swap in a 8 3/4 in my dart now that I have a little more power under the hood. Tired 273 gone, healthier 340 in its place. I know there are a few things I have to do, but the one thing that has me wondering is, can I just figure the length of the driveshaft by subtracting the axle to yoke measurement from my spare B+T driveshaft, and having it cut down by 2.25" and welding back together. I will have a driveshaft place do the work, but can I just give them the measurement to build it, with the original u-joint yoke, and get a dual sized u-joint?