@RangerGoolz ,
If fuel economy is a concern, rebuilding your 1986 2.9l for increased efficiency will deliver both improved fuel economy and the power to get quite a bit more work accomplished.
I needed more from the 2.9l and am shocked both by how much their efficiency can be improved and how more more fuel efficient they can become.
In regards to swapping in an OHV 4.0l, SOHC 4.0l or 5.0l the costs are worth considering. 15-20 years ago, plopping in a junkyard ohv 4.0l probably saved some money verses a proper 2.9l rebuild but today if you want either a junkyard or working doner engine to run decent I feel that at a minimum 1) valve job 2) complete rebuild are on the table.
Comparing the higher cost to adapt an 86 RBV chasis for V8 use with the bolt in ohv 4.0l was once a landslide in favor of the 4.0l. Adding the additional cost of a rebuild, levels the swap cost playing field.
I am not the one to ask about using the SOHC 4.0l, I like using their flat top pistons in the OHV 4.0l for a modest compression boost when a rebuild is already happening.
In the end:
• a built 2.9l is the lightest, requires no structural change and WILL get the highest fuel economy
• the OHV 4.0l is the simplest swap though a rebuild should be expected
• the SOHC 4.0l is reported to be a dependable power plant (I am not a fan of the timing set up)
• the 5.0l will require the most structural adaption, has the greatest after market support, least transmission and exhaust options, best towing and can see 24-25 mpg (highway) if you are willing to do what is necessary to realize it
If you are low on mechanical expertise, then the "proper" 2.9l rebuild will be hard to beat. Considering just cost, fuel economy and end power; you can expect a 2.9l build to cost about as much as a crate replacement engine but yield nearly SOHC 4.0l power with almost 2x the fuel economy of a stock OHV 4.0l. The cost and power for the other engines will increase but the fuel numbers will decrease will ohv 4.0l, even more with either of the other 2 engines.