If you are using top level from long time, then no need to change blogger.com blog on blogspot url and not required custom script inside blogger template.
Just point the IP address of hosting account to top level domain. It will start serving content from hosting account (which is WordPress).
Also, make sure you need to run fix.php
provided in our DIY article and change the permalink, so that it will use the correct url which was on blogger.com
Thanks,