I have an 86 with the 2.9 efi engine. I would like to know the best option for replacing the top end with a carburetor so I can run a duel fuel gas/propane system. Any Options come to mind?

 

None.........the 2.9 is totally EFI dependent, no carb intake exists for it that I am aware of.
If it was a year older truck with the 2.8 carbeurated V-6, you wold be in luck.

 

You don't need a carb setup, you need a propane 'mixer' or whatever they call the pressure reducing rig that can valve gaseous propane into the intake and a switch that will cut power to the fuel pump. The propane doesn't need to come from a carb, it could be tapped into the intake and fed right through the throttle body, or perhaps directly into the intake. Shut off the liquid while going down the road and turn on the propane using a solenoid operated valve. You'd need another valve that could meter the propane as the throttle was operated. Maybe check with the fork-lift and industrial engine companies that offer propane conversions. Maybe even tee into the air intake tube that feeds the throttle body.

 

The 2.8 and 2.9 are basically the same engine. The 2.9 has a bigger bore and is fuel injected but the intakes will swap.