People forget that it was Bastiala standing over Revan in that cut scene when Revan is defeated. We know that she had to fight through A LOT of Sith Acoyltes, Apprentices and possibly Lords to reach Revan on the bridge of his own flagship. She may not have defeated Revan herself, but was apparently no slouch and was able to make it to him with a small stike team.
That being said it would be a close fight against Ventress but I'm going to give the edge to Bastila Shan. Bastila would never have backed down from a fight and most likely would have fought to the death against any Sith or Dark Jedi whereas Ventress would turn tail at the slightest indication she was going to get murked. With her double bladed lightsaber at her side and the force as her ally Bastila would win the day.
I would also like to see a versus with Brianna the Handmaiden, Visas Marr, Lord Scion and Nomi Sunrider all great Jedi/Sith from that era.
Unfortunately for your theory, Jedi and Sith back then would be no match for PT era Jedi and Sith. And Ventress has beaten Jedi Masters in battle. Bastila is really no match for Ventress.