I think factory is 11". I swapped my (used to have) 71 charger with a 10" 2600 stall with 4.10 gear 383 engine. Great light to light. Not great on the highway.
The nearest race track is 4 hours away so my main concern would be getting it to the track rather than ET. I installed a factory high stall when I built the transmission, are they loose enough for a .528?