912 is Fuel Selector

Size the Header tank to your requirements.
It can be as large or small as you wish.
The return flow is always equal to the supply flow Minus what the engine used, so you are Never pressurizing the tank.
It is important that the header tank have a vent line returning back to the main fuel tank.  This is Not optional!

A check valve in the return line very close to the header tank would be useful in an accident scenario if the return hose were severed.
It will serve little purpose if the engine remains running, as the return flow would still be delivering a Liter(Quart) per minute.
Then again, consider that whatever severed the return line might also sever the supply line and you will still be dumping gasoline all over the place.

The odds of ever seeing a return valve save the day are very slim at best.
Keep It Simple, If it doesn't exist, it can't malfunction.

Thanks for everyone's opinion on this subject.  Things to think about.

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Michael