Drop Side, extra blocking?

Yep, the guys running drop sides have blocking they carry

 

Well kind of a gray area, most trailer manufacturers today make a level deck that is as close to same ride height as a drop side. In my opinion the drop sides tend to be stouter and I love them for a lot of things but in certain scenarios they're more trouble than there worth. Your shop forklift for example would suck on one of those (they suck to load in general). As far as far as like your concrete box example I just set them on plywood right on the beam. Some things I've blocked out at the side rails to, it just depends.

 

Here's an example.

 

Unless your going to have a haul that calls for the drop side on a regular basis. I wouldn't bother getting one. A regular deck lowboy will do just fine.

 

If I were to only buy one lowboy I would go with a drop side.You can always add blocking to make a drop side into a flat floor, but you can't make a flat floor into a drop side.

 

Drop side = Cantilever, no I won't call it the former...
Your assumption is correct, a cantilever's purpose is to reduce overall loaded height of drive on/off equipment with sufficient ground clearance.
Quite simply put, they are a massive PITA if that is not >90% of your freight. If you do a lot of drive on/off equipment locally, they are great. If not, they are NOT.
*The rail height of a cantilever is NOT the same height of a flush deck! Read that carefully again.
So NO, when you put blocking out on the deck to flush the deck surface you will NOT be the same height as you would be with a flush deck, you will be HIGHER!
You are already talking freight where this option doesn't sound like the right one for you. You need to consider this, the crossmembers of the deck are way shorter I-beam heights than typical, that means that YES they will flex! So if you have your concrete box blocks on and the deck dips down ever so slightly under load and the center 4' of the floor of your concrete box is now trying to support the weight of the sidewalls and top in the middle unstregthened open portion of the box, how well do you think they'll take that?

Now we've already established that the I-beam cross member height is shorter to build such a low profile deck, let's look at the other side of this equation...the trailer still has a pretty decent load rating right? So how do you think they accomplish that? Hint, they put in waaay more cross members! Yup, you guessed it, they are heavier than typical flush deck trailers.

Your profile shows MN as your location. A cantilever is not really a big need for you there that I'm aware of, I go through MN at 14'6"-15'6" tall frequently with very little issue. They are great for east coast or major metro equipment shuttle outfits where height is critical.

They are a trailer that tries to be everything, but they are only good at one thing, IMHO. They aren't a rail trailer and they aren't a flush deck, don't believe for one second that they are. The deck is in the way for rail freight, and the rail is in the way for most deck freight. The additional tare weight makes it useless when trying to get light with a rail trailer, and all that lovely extra blocking you now need to pack to make the deck flush, now also increases the deck tare weight that you're already sacrificing with all those extra cool low profile crossmembers.

Take a wild guess what I think of cantilevers?

*This may vary with different trailer manufacturers, but the ones I'm familiar with are exactly as I stated. Why you ask? Because the deck portion of a cantilever is much thinner that a typical flush deck, therefore the center frame MUST support the cantilever deck as the outer frame of the deck is also shorter and strength is sacrificed to gain this, so the main frame needs increased strength to compensate. On the trailer pic posted by @johndeere4020, that is an Aspen and their flush deck is 16" thick and the rail of their cantilever is 19.5" on their 50 through 65ton rated trailers, and you require 9" of blocking to "flush" the deck to the rail.

 

I would say your best of both worlds would be a low flat deck. Most 55ton lowboys have a 16" beam, so 22" deck height w/ 6" ground clearance. Dropside decks to vary on the outer beam from 9" to 12" tall, but main beams are typically 16". There are several trailer manufacturers that build a flat deck with a 12" beam, so 18" deck height w/ 6" ground clearance. Stick with your well-respected guys like Trail King, XL Specialized, Fontaine, Talbert...those guys. Only thing with Fontaine is the front connection on the deck sticks out in the way on a lot of small pieces you may have to load. One more thing is if you need to run a single axle stinger then make sure the trailer is built for it. With those short height flat decks there are some guys that will be quad axle only, not 3+1 capable.

 

I pull a 55 ton Fontaine with a 18" inch deck height and where the neck connects it doesn't stick out at all. I load steel drum rollers on it all the time with no problem. I do have flip ramps on the front so that might help.