Wow, when I started typing no else had replied. I must have slow fingers!
Let me be the first to welcome you to The Stovebolt Page!
Pulling that 12,000 pound trailer seems like a pretty important piece to think about as you make a plan.
A trailer that big probably has its own brakes, but if it were me, I'd start by making sure I could stop this thing quickly in an emergency, even if the trailer brakes failed.
So if you start with the brakes you'll probably conclude that the original axles make fitting adequate brakes difficult. And don't forget that a heavier wheel and tire combination impacts the brakes too, running duals will mean you need bigger brakes...
With that heavy a trailer you probably want to stay with a full-floating rear, so you may want to look at some newer rears under 3/4- or 1-ton trucks all ready equiped for duals (its going to be too wide).
The next question is where are you going to pull that trailer, and how fast do you want to move it? A big six with the right gearing will move that trailer, but might not do so at an acceptable speed.
The diesel might be the best idea, but not my preference.
I've been thinking about these issues for a long time. I'm planning to build one of my trucks to pull a trailer with a truck loaded on it. I'm probably looking at half the weight you are, but have decided to power mine with a 454 and use brakes and suspension from a modern 3/4-ton.
I know of at least one guy who has built a truck to manage a big trailer. I'd love to hear from others who have done this, because I want the best information I can get before I start on mine!
