what is best year casted /6?

TXDart said
On slantsix.org they all say to use the 5 freeze plug engine block.
I believe this is the truck block 70's

All the '67-up blocks have five freeze plugs; there are multiple different 5-plug blocks. A "BH" block in good condition is generally considered the stoutest, but most variants of the slant-6 can be successfully hot rodded.

Spend some time over at http://www.slantsix.org/forum searching the engine build threads such as this one.