Not to argue (it's your trailer) but I'd weld it to the platform. The tongue, unless it's made out of the same material thickness of the frame, won't hold as much weight as the frame box will (plus it's further out and is only affixed by welds usually and has a greater disadvantage by the weight arm). If you mount it against the frame box, you can the have the real-estate to mount a locking, angular tool box for chains, boomers and other misc equipment... I'm not trying to be rude in the least, but since you have a golden opportunity to build the near "perfect" trailer I think you'd regret mounting the jack forward on the tongue in the long run.