We looked into putting a 6.9 into a 78 f250 4x4. The only real problem I saw was the ac & heater on the firewall would have to be removed or altered for the engine to fit in the engine compartment . The owner of the truck wasn't willing to do that much modification .

I always say anything is possible with an idea , an immagination ......oh yea , also a cutting torch and welding machine ....lol.