What motors will fit in a 78/79 ford bronco?

For someone that doesn't know a lot about old vehicles, you sure do have some ambitious plans.

The engine bay in a late 70's Bronco is rather large so pretty much any engine will fit. Just how easy that is is relative though.

Ford Motor Company has a large range of V8's in three block patterns, in addition to the 300 Dearborn inline six, that will fit the Bronco. Ye know about the 351 MAC (Michigan Assembly Center), a pushrod V8 redesigned from the 351 Cleveland to fit medium-duty gearboxes; it fits the same bellhousings and clutches as the 429 and 460 Lima big-blocks. But additionally, the 351 Windsor pushrod V8, which shares crank bearings with the 400 MAC, would be a satisfactory update, being built with 4-barrel carburetors through 1987 and sequential multiport fuel injection through 1997. In 1979, if memory serves me correctly, the most powerful engine available for the Bronco was the 400 MAC.

Probably another 351 would be the easiest fit.
Did someone tell you that the 351M in this particular Bronco was junk, or all 351Ms?
I am only familiar with the 351w (made in Winsor Canada) & the 351c (made in Cleveland) & of the 2 of them the351 w is often thought of to be the junk engine, but I've known people with 351Ws that ran okay.
I had always thought that the 351m was a high performance 351, but I'm not sure.
If you start putting in other engines, you will need a transmission that will fit to the bell of the different engine, & no doubt the tail shaft will be longer or shorter than the original, so you will probably need a longer or shorter drive shaft.

Good luck.