Hey there
@bronco78guy welcome to the forum! In time you will get a LOT of great value from this place. Personally, 78-79 Gen 2 is my favorite Bronco look. Someday...
Anyhow, swapping out for wider wheels and bigger tires, I don't know that offset is your most important dimension. That is the measurement of the inside mounting surface to the centerline of the rim. You might want to go with the backspace measurement, from the inside mounting surface to the inner wheel lip. You can measure that yourself pretty accurately, as shown below along with the difference between the two:
A quick Bing search for 1978 Bronco rim offset brought up this link in the third sample:
is 3.75 the normal backspacing to use?
www.broncozone.com
Which validates my "great value from this place" claim up top there.
I'm sure folks better equipped to answer your question will chime in, and probably direct you to the new member introduction section too. Be sure and post some pics of your classic too!