A rule of thumb for driveshafts is 55" ctr/ctr of joint for a gas engine with a 3" tube and .083 wall which nearly all are. Diesels can go to 72" with a 4" tube and 2500 RPM. Set up the OD unit so that the driveshfts going in and coming out of it are the same, approx 2-3 degrees but measure the original angle first so that you maintain it.
