CBC-MAC 8 octet ICV compatable with 16 octet block size?
In general the authentication tag size is indeed generated by simply using the first (leftmost, in most byte array representation) bits. In general Tlen - the length of the tag - is supposed to be a multiple of eight, so a tag size of 64 bits results in the leftmost 8 bytes.

You cannot just change the block size of a block cipher. AES is only supporting a 128 bit / 16 byte block size (although Rijndael supports any block size between 128 and 256 bits inclusive using 32 bit increments). So changing the block size of a cipher is generally out of the question.



The usage of just the first/leftmost bytes out of the full MAC output is so widespread that an AEAD API interface that I wrote based the method signatures on the fact. Note that the output of a MAC is indistinguishable from random anyway, so in principle you can use any bits of the result; using the first/leftmost bytes is however the commonly agreed default.



If unsure: read the standard, NIST SP-800 38C:


  Set $T=\operatorname{MSB}_{Tlen}(Y_r)$


and


  $\operatorname{MSB}_s(X)$ : The bit string consisting of the $s$ left-most bits of the bit string $X$.