Yes 2x6 pin (75W) =1x8 pin (150W).
I'm starting a T7400 project and was looking at the PSU issue. It looks like the T7400 1000W psu will work in the T5400. It has the strange 20 pin MB cable.
The 20 pin cable is 9-12V pins, and 10-GRD pins. So it might be possible to splice it onto some GPU cables on a modular aftermarket PSU.
On page 160 of the T7400 manual they give PSU pinouts But connector 24, and 19 seem to be reversed in the drawing. One is on the GPU cable, and the other is close to the PSU like a MB connector. It provides 2x6pin cables and 1x8 pin cable with the correct GPU pinout. One cable has a 6pin, and the other has a 6 and an 8 pin. I've heard rumors that you can't use both of those(on the same cable) at once. A study of the layout of the separate 12V. rails might reveal something there.It looks like the T7400 1000W is the way to go. It should support a single 300W GPU at the least,
On page 167 of the T5400 manual they have the pinouts there.
They each have a 3rd power connector that is different in each one, and has other than 12V. power to it. I think it's for optional storage solutions. if it's not used then it isn't a problem. I don't have my T7400 yet so I can't be more specific.
This expains the T5400s using GTX1070, and the T7400 using GTX1080. The PSU supports it. If you can resolve the 3rd power cable discrepancy there might be a solution.
I'm wanting to run an HD6970/HD6990 Trifire CFX with 2x CPUs so I'm definitely looking of some GPU cables. 3X8 pin and 1-6 pin. I may have to attempt the aftermarket PSU mod myself.