It's not easy to find proper 4x32GB memory for Mac Pro late 2013. Yes, the max memory capacity is actually 128GB, not 64GB. However, due to "high rank" of those 32GB ram units, the maximum frequency that can run with Mac Pro 6,1's CPU is 1066MHz, instead of 1866MHz. Don't worry, even if the memory is running in lower frequency, the timing is also with lower latency. So you won't sacrifise too much performance.
There are a bunck of memories with different specs. For example, we found it's quite common to see "DDR3 1866 PC3-14900L ECC" memories on Amazon or eBay. Sounds nice, right? Actually it depends. The "L" in "PC3-14900L" means low voltage and may not work on you Mac Pro. Also, it's timing may be improper since we will require it to run at 1066 while it's meant to run at 1866. Also, many memories on the market have metal heatsinks, which won't fit in your Mac Pro 6,1 due to their thickness.
Eventually, we found the best match for you. It works at 1066MHz with 128GB total capacity and best timing. Check our item: