I rather you didn't beat about the bush like that - tell me straight
TB1 carries DisplayPort 1.1 which can drive 4k at 30 Hz. TB2 and TB3 both carry DisplayPort 1.2 which drive 4k at 60Hz. DP 1.2 can also drive 5k using two cables or through multi-stream transport. Which introduces some issues. It is only with DP 1.3 that proper support for 5k at 60 Hz is available, but DP 1.3 is not part of TB3.Which Thunderbolt? TB 1 & 2 run on what looks like Display Port cables. TB1 can not support a 4K monitor; but TB 2 can. But when Apple releases the new Skylake based Macs this year we will see TB 3 and USB 3.1 all running on USB-C ports and cables.
So you might want to consider a plan for replacing the 2008 Mac Pro with a newer machine that does TB 1 to 3, USB 1 to 3.1, all over USB-C ports and cables. The new machines should support external 5K monitors.