There were a lot of changes to the little RBV's over the years.
The 2.8's were carbed. They were a high rpm engine used in the Mustang and Capri for road racing and had a lot of aftermarket go fast stuff. Comon problems were the electronic feedback carb and distributor. Some replace with the earlier Dura Spark dist. and std. 2bl carb. Carb's have trouble in some types of Off Roading. I go to Colorado and it is difficult to get a carb to do well on steep rocky climbs with large changes in elevation.
The 2.9 was fuel injected and had more low end torque. I really like FI for off road. But it can be a lot of work changing all the eletronics over in your truck. Different fuel pumps, gas tank, fuel lines, in addition to the wiring. Not even sure the dash wiring is the same.
As was said if you are thinking about an earlier model 2.9, a 4.0 might be the way to go instead. The early push rod 4.0 has more power and torque. The BII changed again in 89. I have read that the 89-90 dash is more compatable for the 4.0 wiring. Mid 90's pushrod 4.0's are said to be the best by some.
And the 5.0 is an option folks like to do. But that would be none factory. Just depends on how much of a dd you want to keep it. A carbed 302 would be less work and headaches for your 85. The more stuff you want to keep the more headaches. Like AC, .....ect.
The 3.8 is another option. Basicly it's a 5.0 minus 2 cylinders. This gives you a bit more room under the hood.
If it were me I just look around for a newer RBV with a 2.9 or a 4.0 and use it. The early 90's RBV's with the 4.0 had larger, stronger axles. While you are waiting you could try a duraspark conversion and 2bl non feedback carb.
If this is just a toy and you have the time and place to do the work then have at what ever ya like. Your $1.25. Do what ya want.
You need to post more. Can't remember what the number is but once you reach it you will be able to do more stuff like see folks profile, post pic's, send private messages ect. It's that way to prevent abuses.
Good luck and welcome to the board.