This gives you some idea of just how difficult it was to find the seed for pack.png - you simply aren't going to find the seed for a world with only terrain available unless you have access to a large-scale distributed computing project (a system with a single GPU with the average specs of what they used would take 231 days):
This was approximately 93 days of processing time at a total of 54.5 exaFLOPs compressed into the last 24 hours.
Thank you to the 137 users who contributed 181 hosts with 231 GPUs over the last 24 hours. You all accomplished this.
There are other ways to find a seed, such as the coordinates of structures, which is much more practical (2 hours on GTX 1070) and even has a publicly available tool that you can run but you do need to know the exact coordinates (starting chunk) of numerous structures:
Note that this tool is still only half the search; it takes advantage of the fact that there are only 2^48 seeds that determine structure locations, but you still need to search through up to 65536 full 64 bit seeds (more if you got multiple results from the structure search, which means you didn't provide enough locations) to find the correct biome map (they mention a tool called "MCeed-B" that does this automatically but I don't see it listed on their home page).
Even then, it is possible that you will only have the correct biomes locally; as shown in this thread you can find the same biomes (terrain, structures and all) in multiple seeds, though that may not matter: