I havent got all the ins and outs of this but:
8 MnO4 - + 3 S2O32- + 2 H+ --> 6 SO42- + 8 MnO2 + H2O
Should run okay in Acidic Solution, probably have to warm it a bit, to get the Lillac to blackish color change.
As MnO4- is a strong oxidizing agent i doubt that the reaction:
S2O32- -> S4O62- is going to be stable, it will probably be oxidized SO42-
Also, most redox reactions go a lot smoother with added heat...