Heres the suffix code page from this forum. If you have your deck stamp you should be able to decode where your block came from and what its original use was i.e., 340 hp, 400 hp, 425hp etc...
That's the correct 1963 block.
There will a number cast into the block, left side where the bellhousing flange is. 1-365.....is called a Julian date code and is the casting date of the block.