I am unsure about the drop spindles. I think this will take me as low as I want to go.
I have decided to go with a 1990 isuzu 3.9L diesel 4cylinder engine. I have chosen this because they are reliable 300,000 + mile engines in medium duty comercial vehicles that tow 15,000+ pounds every day of their lives so I think it will be good for my 4300# truck. I also chose this engine because guys I know that run them in 6,000# 4x4's get 26-30 mpg so I think I should get at least that maybe around 32mpg or so, plus I can run vegetable oil for practically pennies.

I will be backing this engine with a GM 4 spd automatic trans and the 2006 crown vic rearend withh 3.55 gears and posi traction.
The 2006 crown vic rear axle will have posi traction and 3.55 gears. Originally they used coil springs and a 4 link setup. I am going to use leaf springs in my truck at first and probably go to air bags all the way around later on. For now I will just cut the spring mounts and 4 link bar mounts off the rear axle and then weld on spring perches at the proper angles, after that it will be a bolt in affair.
I will be posting pics of all this I just have to get home and finish the mounts and install the rear axle so I can take the pics, but dont worry its not too far off.