[somerandomguy]

I measured temps in my engine with engine almost at regular operating temperature.
Fuel rail before entering head: 50 celsius
Radiator hose: 70 celsius
Exhaust manifold 80 celsius

I think the best location would be at the exhaust manifold to coil some copper tubing around the exhaust manifold pipe and connect to fuel system delivery line.

Diesel auto ignites at ariund 200 celsius so it will probably be safe. Will see how much mpg improves.

The exhaust manifold pipe also gets hotter and hetas up quicker than the radiator hose.