That leaves the door wide open. AFAIK, there are a number of engines available in 84 F-150. Anything from a 300 straight six to a 460. That means ther are two different trans bolt patterns. The normal small block 300-I6/302/351W/351C, and the 'tall block' 351M/400/429/460. Since the 351C and 351M/400 were gone by then, that leaves the six banger/302/351W small block pattern and the 429/460 pattern. And again, since the 429 was gone by then that leaves just the 460.
OK, so what engine does this mystery 84 F-150 have?
Flat head V-8? You're gonna have to talk to someone a lot older than I am for that one. The last flathead was made sometime back in the 50s. Good luck finding parts for those dinosaurs!
My buddie and I assembled a 50 merc a few years ago.It had a flathead.My wifes cousin owns 2 50 fords with flatheads.He didnt have much trouble finding parts.I see alot of them at the swaps.when you rebuild them no need to use non detergent oil with the new seals.I heard its a fun motor to hop up.
flatheads are returning as the hot item in a hot rod, looks cool also, likes multiple carbs, many parts are made for this old engine including engine to trans swap kits, blowers and fuel injection kits, has a novelty of not being a crate Chevy engine.
Hot Rod magazine (I think) did a feature on a flathead a couple of years ago, I think it was in a land-speed racer. The builder had reversed the flow of the engine (exhaust = intake, intake = exhaust). Apparently this is a semi-common process. There are cam grinders that make this reverse pattern, and manifold makers that supply parts... it was sweet to see headers coming out the top of an engine!
Flat heads are great and today it is possible to get 320 hp out of one and have reasonable longevity. the best rescource you can find is rod and custom magazine. and the best place for parts and service as well as questions is flat head jacks at http://www.flatheadjack.com they are great people and will be able to answer any questions you may have. remember if it has rocker covers it isnt a hot rod!