ORIGINAL: Elouda
M6A did not have a centerline float like the A6M2-N - so all weapons were carried on centerline. Thus Im pretty sure it didnt need a modification to carry even the torpedoes or 800kg bombs.
According to Peattie, it did. (removal of the float(s)). Not alot of info on the plane, so being forced to google [;)], a consistancy that cropt up was that the plane was designed to be either recoverable, or ditched on mission completion (minus the floats). Armament is listed as 'either' a 250kg bomb, or an 800kg load. It would seem that the recoverable loadout would be with the smaller bomb load while the latter would be sans float hence requiring sacrifice of the plane after mission completion. This would track with what Peattie wrote of the plane in his book "Sunburst" (on Japanese naval aviation)