If you overfill it I dont really see any harm coming from it. I might be wrong, but I dont think I am. With other things you will blow seals, like pinion seals on diffs, or all type of other seals if you put to much oil in your motor. With a transmission though it cant build pressure to blow a seal since where the shifter goes in it is not air tight. The worse I could think might happen is that you lose a bunch of oil through the input seal and it contaminates your clutch, even that is a stretch. When you replace the fluid I doubt it will have any problems. Whoever filled it probably filled it by removing the shifter and filling through there without removing the fill plug to make sure they didnt over fill it.
Complete rebuild axle/engine/everything swap in progress:
Parts already gathered for assembly: 2001 Ford 302, 1990 Mustang GT 5.0 ECU and wiring, NP435, NP205, Pair of axles from 04' F350 SuperDuty (Sterling 10.5 Posi rear, Dana 60HP Front-Both Disk Brakes 3.73 Gears), 37" H1 Tires, 9" lift TJ Coils for 2 link conversion in front, Stage 2 Clutch Kit, 3 Row Aluminum Radiator, Chevy Astro Steering box (4" front Stretch), XJ Rear Leaf Springs (6" Rear Stretch), and lots more.