4.0 vs 2.8
 

Is the 4.0 bigger than the 2.8? Trying to decide if I want to go 302 or 4.0 in my 84 bronco ii .
Thanks

Yes. 4.0l is 4 Liters of displacement, displacement is the amount of air an engine can pull in every 2 RPMs
2.8l will be smaller engine, pulls in 2.8 liters of air

302 displacement is in cubic inches instead of Liters, but conversion to liters is 4.94 liters so Ford calls it a 5.0l, 1 liter bigger than 4.0l

The amount of air an engine can pull in is directly related to the amount of fuel it can burn, so the amount of energy it can release every 2 RPMs
Larger displacement means more air so more power

Super and turbo changers add more power to an engine by, in essence, increasing its displacement by FORCING more air into the engine, so it can burn more fuel and release more energy
NOX injection does the same thing, NOX breaks down into Oxygen(air) when released, so more fuel can be added, and more power created

The 1990-2000 4.0l OHV engine was very reliable engine but only available with fuel injection so you have to use a computer and all the wiring that requires

302 can run with distributor and carburetor, like the 2.8l, so often easier to use in a pre-fuel injection vehicle
You can use an AOD automatic(not AODE) or a 1997-2003 M5OD-R2 manual transmission(best shifter placement for B2)
Either trans comes in 2WD or 4x4 versions

I'm actually talking about the physical size. I know on the 302 you have to mod the heater box. So trying to learn whether or not I will have to mod the heater box if I go with the 4.0.

No, the 4.0l OHV is a 60deg V6 engine so narrower than the 90deg V8, 2.8l and 4.0l OHV should be same width and length as they used the same block, should be about 18" wide

If you don't have AC then heater box is not that big of an issue with the 302 V8, this engine is 20" wide, so an inch wider on each side
You do need a remote oil filter and because it sits lower you need to use a electric fan on radiator