Re: 400 vs 460
That's pretty low MPG even for a 400. I had a 351M in my 78 Bronco and thought it to be a dog and got crummy mileage, but always over 12.
As far as the carb? Do you have any other mods? Better intake, bigger cam? If not, you may be over carbed for a 400. The engine can't breathe in enough of the go-juice unless you have also put a new cam on or maybe even new heads. If I recall correctly, the only difference between the 351M and the 400M was the stroke was longer on the 400.
I have never been a fan of the 351M and 400 engines. The valves were small. The cam was detuned, the oil system design was flawed.
Not sure how your F150 is setup, but a 351 does pretty well in a fullsize bronco. If you want gobs-o-power go big.
If the engine is old and tired, I'd consider swapping in a 351W. or if your really into HP, go for a 429 or a 460. in the early 70s, Thunderbirds and some LTDs came with 429s designated "429-4V". The 4V was for 4 barrel heads. The 4V heads had larger valves and larger ports. Very strong motors. My dad had a 73 LTD station wagon with a 429 4V and if you could keep your foot out of the secondaries, it would get 15 to 17 MPG. I still kick myself for letting him give this car away. It had a very strong 429 and a C6 and a ford 9 inch rear end.
If you can find a 315W in a mid 80s bronco, these are much better IMO than the 400M. after 87 or 88 they came with Multiport EFI if you want injection.
99 4Runner (wifes grocery getter)
88 Bronco ** SOLD **
01 TJ Arriving in August