Forget about installing a thermostat with a temp you think may be the right temp setting.
Ask Mr Nissan for a thermostat to suit your vehicle VIN number.
Then ask him what'c the temp setting that's stamped on it?.
Don't quote me, I believe the Gen p/No is 2120042L01W and about $45.
Cavitation from a t piece--- doubt it.
The Y would reduce back pressure for sure, just insure you get the flow direction correct.
that is the stem feeds the y on the hot side too heater and lpg mixer
and the y part feeds back from the heater and lpg mixer to the stem on return .
I tried Y's on GQtb42 lpg, didn't see any difference at the gauge , you may get a deg or two difference if you spend $20 per joint on a Y.
Then again it may allow the water to flow to fast through the lpg and heater cores to allow sufficient time for more heat transfer who knows!.
The speed of flow needs to be just right to allow time to cool off, to fast and water retains more heat.
Again this gets back to the job of the thermostat helping to control flow.
See how you go with the thermostat first hay. cheers.