What kind of data type can be use store a binary bit-stream which is more than 64-bits?
There is no builtin data type that can store 78 bits. However, it depends a bit what you want to store. Like the following cases:


If the 78 bits are one 'entity' (e.g. one really big value), you can store it as an array of bytes (10 bytes of 8 bits = 80 bits, leaving 2 bits unused)
If the 78 bits can be split up in smaller parts, you can:


Create a structure or class to contain the sub elements into fields (instance variables), using for each sub type a normal type (e.g. signed/unsigned 8, 16 or 32 bits).
Create bit fields to save some memory if there are small items and memory space is an issue.
Or a combination of the above



If you have many of these items and you want to save space (thus the 2 unused bits of each 80 bits), you can create a big array that contains many items, and use bit operations to get each item. E.g. the first element will be in the first 10 bytes, where the last 2 bits are not part of the element. The second item will use the remaining 2 bits and the following 78 - 2 = 76 bits, etc. This saves only 2/80 * 100% = 2.5% so only use this when really needed.