I do not have 1.6rc5.
If you are running any version of 1.6 that isn't ST you should download the 1.6ST package and run the upgrader. Once you have done that you should be able to download and install any of the the 1.7 installations and run the upgrader.
I've made some older versions available for people who need them. 1.6ST is available here:
http://tmib.net/files/OST_1.6.zip(http://tmib.net/files/OST_1.6.zip)
1.7ST is available here (if you need it for some reason instead of 1.7.1 or 1.7.1.1):
http://tmib.net/files/osTicket-1.7.0.zip(http://tmib.net/files/osTicket-1.7.0.zip)