Is my '99 33' AVS really a stretched 31 Excel / Viper with an extended swim platform? Looking at Baja Outlaws and Cigs with the chopped back end and bolton swim platforms makes me wonder...

Same cockpit and cabin as 31. Platform is the big difference

... but is the running surface the same?

Running surface is not the same for the obvious reason,,,,steps. That being said, it was the 31 that was made thru the mid to late 90s that was retooled by Larry Smith into the AVS hull. There was one 31 AVS made that I know of, after that the swim platfrom was added and the 33 was born.