When I bought my truck(s), one of the three included in the deal was a 78 F100 with a 460. While it was a chunk of motor, it looked dwarfed by the 6.9! But I will say, there was quite a bit of room around it too!
I'm a firm believer in KISS. To me, the FE is a great engine and real easy to work on in the trucks they came in. The 300 six is even easier. The 460 is crammed in there. If you actually believe there is plenty of room around a 460 in a dentside 2wd ford go change a motor mount then re-evaluate your thoughts. In my older Fords I've used the 4BT Cummins- It is an easier design to work on, lasts longer, doesn't weigh all that less than an IDI, but makes a lot more power and gets better mileage.
NO, NO, NO the pre-79 F100, F150 and F250 2wd trucks have thinner, weaker frame rails than the 4x4 trucks. All Ford 4x4's 65 and newer have had boxed forward frame rails. So yes, a 78-79 Bronco does actually have more beef in it's frame than a 67-79 F250 2wd.
The F350 2wd has a different frame design- Nearly identicle to the 67-77.5 F-250 4x4 frame.
A 4x4 will be easier. The frames are entirely different designs. A 2wd is possible and has been done many times, just doesn't lend itself to the IDI as easily.