First off, put a wrench on the crank pulley and try and turn the engine over.
Next take off the belts and try it again. If it still won’t move even with the belts off it may really be siezed. I have had vehicles with locked up accessories that wouldn’t allow the engine to turn over. A siezed waterpump or alternator is a much easier fix then doing a V8 swap.
If all else fails, get it rolling down the street and pop the clutch in 2nd or 3rd gear.
If it really is a siezed engine, and you are really want the job of doing a V8 conversion, best donor is another bronco. Beyond that, just look at any of the hundreds of posts regarding swapping in anything from LTD engines to truck engines to mustang engines to explorer engines to buying crate engines.
Make sure to match engine balance with both the flexplate balance and the balancer balance. All have to match or send parts to the balance shop and make them match. there are 3 and 4 bolt balancers, standard and reverse waterpumps, plus the short Explorer waterpump. Front and rear sump pans. EFI or carb. Flat or roller cam and the matching cam gear. Exhaust options and limitations if using late model Explorer P-heads. Radiator and core support to deal with. Some of the later blocks have issues with a missing clutch linkage hole.
A lot of this stuff is a pain for most of us to deal with when we have to original V8 right next to us to compare it to. It will become more difficult when you don’t have an original V8 reference to look back to and are trying to figure out how the equilizer bolts to the engine when no one realized that the 6 cylinder equilizer may be slightly different then the V8.
I would get a few restoration parts books and study. Look at stuff that is listed differently between 6 and V8 or more often only listed for V8. That is all the little stuff that is a pain to deal with. Engine and trans are easy to come by, it is the little stuff that will kill you and make the difference between a good swap and a hack job. For that reason, look for the most rotted out, rust bucket, POS bronco you can find for a donor for the little stuff.