Are All 340 Blocks the Same?

I am looking into buying a 340 short block as a backup to my 71 340. The guy is going to get back to me on the numbers on the block, but he thought it was a 72 or 73. I believe the compression for the 71 is 10 1/4 and it dropped to 8 1/2 in 72. He didn't think it had the stock pistions, anyway. If I ever had to would all my parts fit right in or bolt on the 72 or 73? Would the crank and rods be the same and the 71 heads bolt right on?

Thanks.

I had to read your post 3 times. Please hit enter at the end of a sentance.
Any ways a 71 is internally balanced.
Some 72 are external balanced.
All 73 are, which are easy to see it says cast crank on the balancer.