The standard GUI is intended for 4 byte blocks, so that is why you are getting the long data error. You could use the test tab of the GUI and set up your command strings as required for 8 byte blocks. See the app note below for the host protocol used for ISO 15693. Section 5.4 shows the write single block command. You could use something like this, but extend to 8 bytes. Be sure that you are also extending the length of the bytes to be transmitted. For example, a write of block 0x06 should look like the string below. I don't have one of these tags here to test with, so let me know if this works out for you.
0113000304184021060403020106040302010000
40 = Flags
21 = Write Single Block Command followed by data
ISO 15693 host protocol: http://www.ti.com/lit/an/sloa141/sloa141.pdf