I don't think I've ever seen a specific "kit" put together for this. There are some speed parts that can help put it together. From what I've seen those are centered more around race use and omit a thermostat all together.
If this is going in a street car, you want/need a thermostat.
It's a little bit more involved, but take a look at this video from Fast Monty. What he did on his engine is probably the best you can do for a street car specifically.
https://www.youtube.com/watch?v=v_qUY4E33r0&t=188s
Long story short, yes tying both heads together at the back to move fluid through the core and back to front of the engine, does help reduce temps at the back of the heads. However, you really need to dump the coolant into the cross-over, not back into the pump where you're adding that hotter fluid to the rest of the engine, bypassing the radiator. This is the same reason AC equipped cars come with heater block-off valves. It allows the condensor to work better and doesn't shove hot fluid back to the front of the engine.
This requires welding a bung to the cross-over to achieve.
I will say however, that unless you're doing sustained higher load, higher rpm driving, like open road racing, circle track etc. There's probably not a huge benefit in doing so to save 10-15 degrees at the back cylinders. What I do personally is tune around those cylinders specifically.