Most importsnt, what is your required accuracy - NOT of the drawing! - but of the four finsihed assemblies as they aim into the projected intersection zone?
If you will, the only appropriate question is: How accurate must the four "pipes" ?? be to aim their four beams (??) so the beams hit each other?
When I have done this machining and welding (the distances were much longer at 10-12 feet, but the "beams" were IR cameras that had to be aimed at the blade roots of a combustion turbine. Thus, the "camera aim points" could be as much as 1/4 off and still produce a useable image for the testing.
I found that laying out the holes and the drill points through our 1-1/2 thick steel exhaust walls was difficult enough, but the hardest thing to do was "aiming" the centerline ofthe camera tubes at the target point and positioning each tube for the tack welds. Once tacked in place, then the welding had to be carefully maintained so shrink and draw movements from the weld heat didn't distort the aim point.
I solved the "aiming" by getting a laser mounted on the camera housing centerlines with a temporary jig and fixture, then aiming the camera tube with one person inside providing directions by radio to the welder and "laser aimer" outside with a fourth holding the tube in place by hand for the tack welding. Once tack welded, the finish welding didn't require the extra hands.
You will need a fixture or target precisely positioned so the laser beams can actually "hit" that mid-air aiming point simultaneously. Now, if all four lasers hit each other, but the aiming point is not exactly right, will that result be "good enough"? My laser beams were 1/8 diameter - will that diameter beam be close enough to allow you to aim the tubes or drilling bores? You can mount the laser inside the drill chuck to verify where the drill itself is going to be aimed once the gadget is mounted in the vise/machine tool table. A 3D CAD/CAM mount might work as well, but expect several trial and setups.
2. How do proposed to install and build the gadget with those holes and intersections with the outside wall?
3. You have a mixture of ANSI dimensions/callout for pipe threads and metric dimensions: Do you really want to do that? It will cause problems?
4. A screwed thread 1/4-18 NPT will NOT precisely position the center instrument vertically: Do you really want to use normal pipe threads for that fixture?
5 I don't think that "8" (mm ???) deep is not enough engagement for regular pipe threads: They will likely bottom out the threaded fixture. Unless that is your intent - but if so, then the fixture will not be tight in place.
