I have been successful with classic 6.5 directly to ubbthreads7.6.0. using the import script.
The site did not have any modifications to the code
But for some reason no success with polls.
I had to do it off line with xampp-win32-1.7.3 because the host was using php 5.6 and I kept getting errors on the import script. and I could not downgrade php due to other php software used on the site.
Then just transfer the files and database to the host..
Update settings in config.inc.php
The big annoyance is many shared hosting accounts have limits on how big a db they allow to import.
You can have a large database and usually you can download it but upload is the item of concern.
Some hosts will import the database for you. Or you can try tools like SQLDumpSplitter or bigdump
Iit has always been suggested many times to upgrade to the last version Classic 6-7-3 first, Then run the import script to get to ubbThreads.
I assume the reason is to cleanup the Classic files for import, due to misc. reasons like compatibility with the import script and mods.