904 Shift Problems

I have a 75 Duster with a 318 w/ a 904 trans. When I got the car about 6 months ago it was a rocket considering only having a 318. One day I went to a small burn out contest with some guys I grew up with. Since that day the trans has been goofy. Now this was not a high rpm burnout or a long burn out, I really didnt want to fry the new tires, so Im sure the trans isnt blown. But anyway, the issue was the car would no longer shift into second. and I had to wind her out to get her in third. So I knew something was wrong, I talked to some guys, they looked at it, and suggested I put a spring to pull the kick down linkage forward. Now I know nothing about transmissions, I have built motors from block up, but I never got into trans work. So I took their advice. Now I have really week and quick shifts, the trans will shift from second almost instantly into third with out building any rpm in second gear. So in town and just normal driving around, the car now drives so sluggish it is destroying my nerves. Can someone please maybe explain what may have happened or what I can do to fix this problem.