My son has a 1990 Ranger 3.0 gas motor with bad heads and I'm just going to do a motor swap. I can't find a gas motor anywhere near me in southern NH and the closest available motor is down in NJ. I can easily get my hands on a good flex fuel motor but I've been told I'd have to swap a few things around to make the flex fuel motor work such as the manifold and wiring harnesses.
Any insight and knowledge to the matter is truly appreciated.

 

Welcome to the forum

Unless you get a 1990 to 1994 3.0l Ranger engine you will need to change upper and lower intakes, so you will NOT have to change wiring harness, thats why you do it so engine wiring matches

Flex fuel engines run fine on gas only thats not an issue, but they will be 1995 or later model so you will need to change upper and lower intakes to match sensor placement and fuel injector size.
Flex fuel uses larger injectors and 1990 computer would be opening them too long at first, so Rich running, it would adjust to them after engine warmed up and O2 sensors started to work, but then it would set a Rich Code because of the wrong size injectors, so CEL(check engine light) would stay on, basically forever, or until you change to correct injectors size for that computer.
E85 fuel has less power/energy than gasoline, so more E85 must be added via the fuel injectors, this is why flex fuel engines needed larger injectors.
And the Flex Fuel head chambers had slightly different shape for the slower burning E85.
But you can run E85 in pretty much any MAF based fuel injected engine as is, computer will compensate for different Air/Fuel ratios after warm up, but can get Lean codes with the gas only injector size

 

Thank you very much for the insightful knowledge. It'll definitely help.