Is it standard practice to cantilever heavy loads out over the cab?

The short answer is that extending a load past the cab is common, but the legality is highly dependent on the state and local regulations regarding maximum load length and visibility.

Since you are dealing with aluminum beams, they are long and rigid, wich means they are prone to shifting if not secured properly. You have to treat that overhang like a critical safety component, not just a place to park stuff for a minute. I've dealt with hauling lengths of steel and aluminum for framing jobs, and the biggest issue is the load moving when you brake or turn. To secure it, you need more than just a couple of chains. If the beams are resting parallel to the truck bed, use straps or chains anchored to both the bed rails and designated frame points behind the cab. You should have at least two heavy-duty tie-downs at the end of the load, even if the load extends past the cab, to prevent longitudinal shifting. Consider using heavy blocks or lumber placed perpendicular to the load near the cab to keep the forward movement in check. Regarding the overhang itself, you need to make sure there is absolutely zero risk of it swinging or catching anything. If the beams are high enough that they could hit a low bridge or a power line, you definatly need to use specialized load covers or banners that extend past the load to give other drivers warning. Also, check your local ordinances for 'oversize' or 'oversized' limits, as those rules will dictate if you can even run that length on public roads without a permit. If the beams are just resting on the ground and you are driving in a construction zone or on private property, the security is easier, but if you are on public roads, treat it like a high-stakes moving operation.

I've never really hauled anything that far out past the cab before, just smaller things for a neighbor's project. It feels super unstable when you think about it. I'm mostly worried about how the truck handles when I pull a turn. If the load is unbalanced like that, does securing the end help prevent the whole truck from leaning or pulling in one direction? I'm trying to figure out the best way to distribute the weight along the frame.

I totally agree with the point about anchoring the tie downs to the frame points instead of just the bed rails. You are right, those beams are heavy and rigid, so if you rely on the rails, the load can just shift laterally. When I pulled 24-foot runs of conduit for a commercial project, I found that if the load wasn't perfectly centered over the truck, the whole thing whould pull hard to one side in sharp corners. I always made sure the load was perfectly level before I ever started securing it to the truck.

I hauled massive pieces from the scrap yard years ago. The biggest mistake I see is just letting the metal sit directly on the truck bed. You gotta use proper dunnage, like stacked lumber or heavy plywood under the base of the load. That stops the material from flexing or sliding around when the truck hits a bump or a dip in the road. If you just secure it and ride over gravel, you might as well toss the keys.

When I was pulling long pieces of sheet metal for a project, the thing that made the difference for me wasn't just the number of straps, but the type. I used heavy grade 8 shackles rated for maximum tension at both ends of the overhang. That made sure the load stayed flush and didn't start swaying when I hit rough pavement. If you just rely on the basic D-ring hooks, they just aren't rigid enough for that length of aluminum, especially if it's prone to flexing. You want that anchor point to be solid, because the movement of that heavy stuff is what causes the biggest headaches down the road.

Update: job's done, beams made it there fine. Took the advice about anchoring to the frame points instead of the bed rails, and put scrap 2x4s under the load like one of you suggested to keep them off the bare bed. Flag on the back, checked them at a stop halfway there and nothing had budged. Appreciate the help.