Looking at a 340 block, be aware if the block is standard bore, or has been bored. General consensus is a 60 " bore is the max. Sonic check any a block u are considering. Street use, the engine should be ok with a 60" bore or less. People have bored them out to 80" w/o trouble but I would avoid them.
Just my thoughts!
All 340 blocks are equally good they just went cheap with the cast cranks and small valves in the latter engines, the T/A blocks are thicker around the pan rails for 4 bolt main conversions so they are the best production car block then you had the X block of the late 70's this was the best block of all.