LY 6 waterflow

I need some info on a LY6 waterflow diagram.
My heater is not getting really hot like previous years on the 5.7 (monsoon340)engine.
I normally tap near or at the thermostat and return to a Y-Pipe.It gives plenty of heat but most important the LPG converter gets enough hot water to vaporize the liquid lpg.
Now i have the LY 6 and the 400 hp needs 2 converters/regulators with the current set-up(tapping from the drainhoses and return to Y-Pipe)it seems there is not enough flow.
The heater hose is 160 but the air comming out of the heater is not real hot.

would drilling a bigger hole inside the y-pipe help ?
or does someone has a better route?
here are some pics of the thermostat housing.
i could really need some advise.

My 2007 x2 MCX was the same way and had the same problem. Dealer moved tap point to hose coming off manifold. The drain hose tap doesn't give you the flow you need.

The problem is definitely water flow not being high enough. You need a bigger pressure differential. The highest pressure for hot is before the thermostat. The lowest pressure for return is closest to the waterpump impellor.
I'm not familiar enough with that exact motor to give you more help.

Tap point to hose coming off intake mainifold.

I am totally unfamiliar with heaters BUT if you heater is supplied off of the U shaped rubber hose highlighted in pic 2 and lower left in pic 3, I would say the hose may be kinking. Especially as it gets hotter and the hose becomes softer. Just something to check, if I am off dis-regaurd my post and I apologize. Just trying to help!!

any help is welcome,..i,m so confused on this one