Different people will tell you different things. There's a well known issue with the casting on the ST205 block, the only way to tell if you've got one which will put up with abuse is to get it tested. I think they use sonar or x-ray or something bizarre.
There were rumours that there was a thin-wall and a thick-wall block but these seem to be just that - rumours. More likely is that due to the way they are cast, some come out with thicker walls than others and will cope with higher boost and temperatures.
Also, the supposedly 'ceramic coated' pistons in the ST205 are more prone to detonation that the previous two generations.
As far as the strongest, that's well argued but some suspect the ST165 was the strongest. Problem is there's seldom few available now in good enough condition to tune heavily and I don't think (IIRC) that Toyota sell that block new any more.
You will see over 300bhp on stock ST205 internals, but 350bhp would be pushing it to the limit. There was a car imported from Japan that was running high figures (I think around 400bhp) with lots of unknown mods and tuning. And when it was eventually opened up, was stock inside, so it is possible. Can't remember the guys name now though, was several years ago.