That PSU is pretty confusing.
On the one hand, it has 18A on the 12V rail =216W
BUT,
It states the max on the 3.3V + 5V Rails is 160W
And the Max of the unit itself is 300W
To me, that would suggest (at best) a little over 11A on the 12V rail to = ~140W.
I'm assuming that's the original, stock PSU of the i3-550 unit? So from somepoint in 2010?
Add degration to that PSU over time...... I'd be surprised if it was stable at significant power draws*
*The 1050TI isn't going to push close to 75W though, when paired with the i3....
It'll be really close, IMO.