Mark, I, too, like Option One. Only ... Consider the 4BT instead of the 6BT. The 6BT is a pretty big engine for the Advance Design truck and requires significant modification to make it fit. The 4BT is a far easier upgrade requiring far fewer mods to make it fit. I would think that with the right transmission and rear axle combo, a 4BT would have plenty of torque and power to pull that trailer. One would, of course, also want to upgrade the braking system ... BUT ... if you are upgrading the rear axle to, say, a Dana 60 or similar, you could find rear disc brakes ... and a corresponding front axle mod (to match the rear as far as lugs/wheels.tires/etc) to complete ... and I think, even in the real world, you'd have a capable tow rig for that trailer.
At my day job, we have several pieces of equipment powered by 4BTs and the power/capability they have is impressive. The truck I mainly drive for work is powered by a "Big Cam 400" Cummins which is the Big Daddy to the 6BT (and Grandpappy, I guess, to the 4BT). I am a big Cummins fan!
At several shows in our region, we have run across several Advance Design smaller trucks powered by 4BTs and Detroit 4-53T's -- very nicely done mods on original chassis. Considering the number of bread vans out there that were powered by 4BTs and had automatic transmissions ... I would think that that combo would be very plausible and realistic.
And doing the Dana 60 or even an 80 axle swap (with corresponding narrowed front axle to get disc brakes all around), although expensive and requiring skill in the real world, is very plausible as well.
Good plan!
