The Tascam 788 is not a cassette based machine. It is a digital machine which records to a hard drive.
And as far as slaving it to ProTools, doesn't ProTools allow you to set a project tempo before, after or during the tracking process? The fastest way would be to just take the tracks 'wild' --- meaning no time code reference at all --- then set the tempo after you have all the tracks in ProTools. You should be able to zoom in enough to allow the bar markers to be a guide. If you are going to be printing further tracks alongside the 788 tracks it will need to be more precise than if you're just doing a mixdown...