It depends on wheel backspacing and width, tire width, and how much trimming you're willing to do. For the most part though, 35's with no problems, 37's with a little trimming. 73-79's have massive wheel wells.

 

I fit 35x12.5-17 with 4.5" BS wheels on my 72 f250 stock height no rubbing.
My 79 F350 ( which is not a highboy) rubs with 33x12.5-16.5 3" BS wheels but does not rub with 265/75-16 which is about a 34x9.5 on stock steelies which are 5" bs... no rubbing.

A pavement princess though will be a whole other story, if its not a pavement princess then WTF... Sawzall dude, 40's here we come...