I have an83' Monte Carlo I have been running on a local track and last week noticed my right side wheelbase is 3/4" less than my left side. Are there any ideas as to how to lengthen the right side and shorten the left?  

I may have some help.  First I am going to assume you are measuring from wheel to wheel at ride height of course.  If this is the case it may not be the most accurate way of getting the rear steer or for that matter getting the rear end straight.  If you have your front caster already set, the spindle is going to move toward the rear of the car, and the right side spindle should most certainly move back farther than the left side spindle which is probably why you are getting the bigger measurment on the left side.  What I do is block up the car and suspendion at ride height.  Then I will measure from the center of the left front ball joint to a point on the right rear.  Then from the right front ball joint to the similar point on the left rear.  I bet when you measure it it will be less than 3/4".  Maybe 1/2".  Which meanse you may only need to slot one hole on either side about 1/4" because moving one side of the rear end back will also move the other side ahead.  Like I said I usually will slot the hole on the rear end bracket with a die grinder then I will bolt the trailing arm back on with the rear end slid back in the newly ground hole and tighten it up to do a test measure again.  If it looks good weld some nice thick hard washers on both sides of the brackets and your done.  If not keep griding till its straight or you get the desired of amount of rear steer you think you need to tighten you up or loosen you up.  Hope this helps.