[vettestr]

These cars are about as aerodynamic as a brick and at 90 " a real hand full. All good advise already but an old trick for making go straight is preload the chassis for torque if a 1/4 track car only. A solid rear makes this easier. To add preload mount the rear diff at an angle to the track or raise the right side axle center 5/8 ". This is not affecting pinion angle final - preload.
Easy way is to find center of rear housing L to R and mark it. Now place a 1" deep socket on its side placed on a floor jack. Slide the floor jack and socket right under your center mark on the bottom of the rear diff housing (front wheels and suspension is on the ground or loaded. Now jack up the car from the center of the rear diff housing. You want the right side rear tire to be off the ground 5/8 " more or higher than the left rear tire. If the tires are round and the same circum. it will go straight as an arrow with 5/8 " preload. Put a good cage in to keep you safe and things from moving around.