The best place to put the probe would be in the bare 2" pipe nipple between the manifold and the mixing elbow. I didn't think there would be enough "meat" in the nipple to tap it for the probe, so I would need to weld on a short pipe collar. I didn't want to weld in the boat, so I removed the four bolts from the flange and removed the whole assembly.
As is so often the case, once I got a look at the parts, I decided that it would be a good idea to replace everything with new. The elbow, which looked OK on the outside, was pretty scaled-up inside, and thin in some places too. I checked, and I found that it is still available from Buck-Algonquin, and not too pricey, but I thought it might be fun to make my own from stainless.
I found everything I would need online in 316 SS. A 2" 45 deg threaded elbow, a 3" 45 deg weld-joint elbow, a 1/2" 90 deg street elbow for the injector, a 3/4" street elbow for the hose connection, and two 1/4" weld-on collars.
A bit of cutting, fitting, drilling, and then some MIG welding, and we had a complete unit. I'm out of practice on SS welding, so my beads won't win any awards, but they are strong and functional. As this elbow is now completely jacketed, it needs a drain point on the bottom, and that was what the second collar was for.
It looks so nice now I hate to put that nasty old exhaust hose on it, I might have to replace that now too!
Tom