what is best year casted /6?

198 rods are very difficult to find and tend to be very expensive if you do find them (if the owner is aware of this)...Not worth it IMO.

As far as building a strong /6, about any will do. Some even prefer to build the cast crank engines because they are lighter. I like the BH block and it is said to be the strongest (note the previous comments regarding the 5 freeze plug block) and I see it commonly associated with 2806830. Early 70's work for me.

I have a few blocks and I weigh them the see which one is the lightest. I have one particular 198 block that is ten pounds lighter than my 225 block.