I would generically answer your question.
To my knowledge AIX 5.3 supports 64 bits. It appears that somefiles from the installation kit are not acceptabed by the AIX box or somefiles on the AIX box itself could be in the prev version, not re-compiled etc..
Suggest you try it on a dev box both with 32 bits and 64 bits and observe the behaviour of AIX. Also you can figure out if any items are on 32 bit which may need re-compiling and bundling.