904 slow to engage in reverse

Hey folks, Long story short ... I have the chance to buy back the 71 duster I sold about 4 years ago. When I owned the car I had the transmission rebuilt with a transgo shift kit added. After it was installed I put about 1000 miles on the car with no issues. Last weekend I go to look at the car. It shifts great, goes through the forward gears with no issues, When I put the car into reverse you can't feel it engage, if you quickly press the gas reverse will engage harshly after you get the RPM's up and then it backs up normally. If you ease into the pedal reverse slowly engages, and again carries you backward as expected. Same results whether the trans is cool or hot.

Any ideas on what I'm getting into here? My first thought was to change the fluid, filter, and adjust the reverse band. He's only put 600 miles on the car, and he said it started about a year ago. I don't think he's abused the car.

Thanks!