Mine is quite similiar to this build as well. I have a 24 x 24 x 1/4" steel firebox, plus charcoal basket, plus ash pan all behind the axles. I used an old boat trailer and had to move the axles as far back as I could get them. Basically as long as the tongue is well grounded (it takes alot of effort to lift the tongue end into the air), you should be fine. Plus since mounting the smoker, I have added a few other things to the front of my rig. Mine pulls just fine. So far I have probably logged over 500 miles behind my truck. I was in good shape where we had the axle at first, centered with the cook chamber, until I inserted the basket and pan, then the tongue just went up into the air until the bottom of my firebox hit concrete. No damage done, just loosened u-bolts and moved back.
I would seriously, if you're not already, consider adding an upper rack. It will run about 30-40 degrees hotter than the lower rack, perfect place for smoked burgers and chicken (just not above other meat), sausage and hot dogs. It will nearly double your Q-pacity. And don't forget about adding some anti-tip pieces to your slides to keep your grating form taking a tumble forward if you pull it out too far.