I never got the longer probe.
I forget the exact length of the probe that came with the VDO Vision gauge kit (look back through this thread I think I mention it), but after talking to lots of diesel guys both in and outside Defender Source, including marine diesel specialists, the universal conclusion is that the tip of the probe does not necessarily have to be in the center of the exhaust flow to obtain an accurate reading. It's more important to get the probe close to the head as possible (i.e. pre-turbo). The EGT blank plate on a 300TDI mani is perfect.
I think if you wanted an instantaneous reading, then yes, being in the center is important, but there is probably a 5-10 second delay anyway (with the VDO probe). There are probes that are thinner and provide instant feedback, but that's just not necessary in this application.
I've logged plenty of miles and heavy WOT runs in my truck since the EGT probe install and injector pump tune and the readings have been very consistent. I have not verified with an IR temp gauge, but I'm pretty satisfied that the VDO gauge probe I've used is providing accurate readings even though the tip is not in the center of the exhaust flow.
I will say that when you weld the bung adapter onto the blanking plate, make sure it's at that angle (not perpendicular to the face of the plate) and that you drill the hole in the blanking plat wide enough so that the bung can be sunk into the plate as far as possible to give you the most penetration into the manifold.
You will be fine. Good luck
EDIT: just quickly went back through the thread and I should add that Uncle Douglas was correct and that the fact I was only seeing temps in the 450-500 range was because my IP was lean and I hadn't put some adjustments on the IP. Ever since a trial and error period and many adjustments, we are running around 650 around town and 750-950 on highway WOT, full load for extended period will see 1,050 before the fact that you're simply going too fast for a Defender makes you slow down.