Engine Swap Question

Conventional U-joints don't rotate smoothly at any angle other than straight ahead, i.e they actually speed up and slow down slightly as they rotate, setting up a torsional vibration that varies with joint angle, so you want to phase the joints to cancel the effect out as much as you can to limit the vibration. To do this the two angles (output shaft/front joint/driveshaft and driveshaft/rear joint/diff yoke) should be as close to the same as possible, so the trans output and diff input should be parallel and stay parallel as much as possible as the rear end articulates....if you want/need a "straight through" driveshaft to the diff, you can use a CV joint behind the trans to minimize the vibration. Get his wrong and you'll have annoying vibration and in some cases like an old aluminum driveshaft, breakage. It was a big problem on my short track cars with locked diffs pulling really short gears, at least until an experienced guy showed it to me.