from experience and research, the point of the return flow to the other heater port is to have the coolant circulate to the main thermostat and have that thermostat operate as designed, not the one that seals the heater circuit.
experience:
on my LS1 swapped 5 series BMW, heater circuit is plumbed into the factory water valve, it stops flow to heater core. ECT would creep up and get hot 225+, i caught it and then turned the heater on, which activates the valve and lets coolant flow.. boom, ECT dropped nearly immediately. I just left the passenger heater turned on... later I did the research on the why...
research:
when the heater core circuit reaches temp, and opens, it seems it is not enough for the engine to properly cool down, so hot coolant stays circulating in the engine v.s. flowing out of the thermostat.
a super nice compact option that i haven't tried is:
https://www.lojkits.com/products/ls-...r-bypass-block
I special ordered some fittings from Lowes and cobbled up my own variant using pex pipe adapters and a coupler on my bmw x5 l83 swap:
https://www.bimmerforums.com/forum/s...7#post30627447
I got creative and used a bolt that fit in the drill and then used electrical tape to sneak up on the size to fit inside the "tee" and just used a file to get the ribs off so a coupler could snugly fit on then used plumbing solder to connect it all together.
then you can use the vintage air shut off valve no problem, or a valve to block flow to heater core to increase a/c cooling.
I have a Stant branded 160º thermostat for one of my swaps that I ordered for the wrong water pump design...(there's 3 styles) and the disc that seals the heater circuit (back of the water pump housing, has 4 holes, maybe 3/16 diameter... maybe some people are using that style thermostat and blocking off the heater circuit and not running into a problem?
On the L84 water pump thermostat, which is the same as LS3, you probably could just remove the cir-clip and that disc all together if you are looking to block off the water pump heater circuit 100%
clint