Transmission slipping when hot?

As I was practicing launching (preparing to drag race the Duster for the first time), I noticed that my transmission seems to "slip" a little or sometimes a lot between 2nd and 3rd gear, but only after several runs.

Driving to the track on Wednesday night, I was stuck in awful traffic, just poking along (in drive) for 45 minutes.

My first run (right off the highway)... I get up there and stage, and take off, and it feels like someone has his foot on the clutch a little. Except it's an automatic. More exaggerated slipping from 2nd to 3rd (at wide open throttle).

Second run, I feathered the gas a little (starting in 2 and then shifting to D manually) and it was better but still a good slip from 2 to 3. The car had been sitting a while and cooling down.

Third run, almost no slipping - really cooled down (45 degrees outside) but still a little between 2 and 3.

Here are some details:
1973 Duster
904 transmission, all stock
stock rear end
Transmission leaks fluid toward the front (haven not opened it up)
For some reason, never have been able to shift all the way to low - only 2 and drive. In fact, I never needed low before.

I was sort of hoping not to destroy anything on the drag strip but here we are.

Thoughts? Is this something I should even take on?