With a stock pan, a pan gasket replacement usually requires 4 quarts depending on how long it sat after shutdown for a refill and, yes, you can mix fluids .
Personally, I would drain the converter also and adjust the bands, especially if it hasn't been serviced in the 20+ yrs. you have owned it.
Tim
'71 Charger 383/727 '17 Challenger SXT (Wifeys car )
)
