Easy for someone who has done this stuff before. For someone doing there first car project, it is pretty major. For starters there is the cutting the engine perches off the frame and welding new ones on in the right place.
Beyond that you are looking at a whole bunch of little adaption projects. These are more involved than the normal auto repair of simply taking something off and putting a new version back on. It is a matter of making something that didn't fit before and having it fit now. Throttle linkage, cooling, electrical, oil pan, the list goes on and on. While none is impossible, there are a lot of little things.
And when you say a C4, I can only hope that you have already sourced a Bronco C4 that is ready for a transfer case. Because if you are starting with something else, that adds a new level to the complexity. Just doing a C4 conversion to an existing V8 Bronco is a fairly good project all by itself. Having to install the adaptor kit, I'll just state that if you are asking if putting a V8 in is easy than installing the transfer case adaptor (and the transmission rebuild to go with it) is probably too much.
