[vettestr]

Do a little web surffin and read about pinion angle and how to measure. Rather than my description of step by step go to Alston Chassis or several other sites to read the pro's.
You need to laod chassis and then check pinion angle, then check driveshaft angle.
Engine and tranny as mounted in chassis is pretty much a factor determined by manufacturer of chassis. The rear is then mounted in chassis and adjusted to proper relationship with 4 link or what ever your means as purchased/designed, if you can not obtain the proper pinion angle and centering in frame then that problem must be corrected.
All centerlines and angles can be checked with good old plumb bob, level and angle finder tool. The new lasers built into many levels make the job much easier but still takes time and patience.
Adding preload to the rear will make it go straight under hard throttle but too much makes little things like turning a bit twitchy.
Quick track check is to place a 1" deep socket on its side on top of jack lifting pad. Now place jack in the center of rear differential housing and start to jack up car. If centered and lifting car with sockets round surface contacting or lifting the car it is easy for weight to pivot on socket. Right rear wheel should be 1/4 inch off the ground before left tire lifts. With 1 more jack pump you now have both rear wheels off the ground teetering on socket.