Convert MyISAM to InnoDB
InnoDB tables tend to occupy more space than MyISAM tables do.
While converting, new physical files are being created, so you have to have free space on the disk (Hard to estimate, but to be safe I would say 120G)  
If you use Percona tools to make the change online, all modifications on the original table will be stored in a temporary table using triggers. So depending on the DML operations,i.e. inserts/updates/deletes, you may need more space. Be careful that on high traffic tables, triggers may affect the performance.
It could be that some InnoDB related configuration should be tuned. 


If in your setup you have two or more servers, like master-master, or master-slave(s), I think it is better to do it offline, one server at a time.