phpnuke7+phpbb2->phpnuke8.x+phpbb3 upgrade

I've a phpnuke based community site that's been broken by hackers over the years and though I've hardened it by changing some of the PHP, it really needs to be upgraded. I'd like to try upgrade it to phpnuke8.x, phpbb3 and reinstall the gallery integration that's in place. I wiped the gallery DB accidentally trying to do an upgrade, but still have all the old photos and would like them re-imported into gallery.