Why not simply build the crane in the form of an I-beam, to level things out. And then attach your U-bolt to it's forward end? It would be stronger, & by properly attaching said crane to the masthead, the normal way with bolts, you'd be avoiding using nuts as spacers.
I'm guessing his reluctance to build it your way is because there are strong sheering loads on spinnaker
cranes, not just loads pulling on them in the vertical. So mounting things in the fashion which you describe is putting unnecessary loads on the masthead/crane fasteners. And in particular, on the threaded sections of the bolts, which is where, they are, by far, at their weakest.
If his concerns are about strength, it's likely because a kite is the one sail onboard which is FAR more likely to induce a full knockdown of the boat. Which makes for a lot of load on that one fitting.
Also, BTW, why does the crane need to be level to begin with? And if you're paying him $ for his expertise, why ignore it?
If you like, do as is common in any other field. Get a 2nd opinion, running all of these ideas, discretely by another rigger or three.
PS: Consider just bolting the U-bolt in place, through a piece of the crane which has been doubled up/thickened. That way you can change out the U-bolt periodically with just a wrench, from the bosun's chair. As opposed to having to drop the rig when a welded one shears off. And then having to hire both an expert with a torch to cut away the mess. And a machinist, to make you some new holes to start over with.
Also, a crane for a kite halyard is something where you want Far more strenght in things, than in most other apps, relatively speaking. So the less work which is needed with a torch the better, as all that heat, alters the metal at the molecular level. Though that can be compensated for.