Gtx 760 recommended psu size is 500w with 2x 6pin pcie connectors. Your 400w psu will not have that and it's not recommended to use adapters.
If you figure average pc's pull @150-200w and that gpu is capable of upto 225w, you see the problem with a 400w psu. There's many 400w psus that realistically are only worth 300w at max, further making the use of a 760 highly questionable.
Make and model of the PSU as well as it's age? This article states you're going to need 500W of power from the PSU for the entire system once you populate the system with a GTX760.
Manufacturer recommendations/ “requirements” need to consider the rest of the system, the PSU quality, and if the PSU can really deliver the rated amount, hence why their recommendations are high.
You may not have sufficient connectors either.
Gtx 760 recommended psu size is 500w with 2x 6pin pcie connectors. Your 400w psu will not have that and it's not recommended to use adapters.
If you figure average pc's pull @150-200w and that gpu is capable of upto 225w, you see the problem with a 400w psu. There's many 400w psus that realistically are only worth 300w at max, further making the use of a 760 highly questionable.