I'm looking to buy a 1971 Ford F100 with a non running 302 in it. My question is, what modifications would be needed to fit a 1973 360 in the 71?

F100's could be had from the factory with 360's, so mods would be minimal. Are you going with this 360 because you already have it? It's the same block as a 390, which is a better way to go than a 360. Honestly the 302 is a better engine than the 360, some people will think you're crazy to make this swap, unless the 302 is beyond salvation.

360 was a good engine, but they were gas hogs. I would stick to a Ford small block such as a 302 or 351W. If you must use an "FE" series engine go with a 390 or a 428.

I'm not sure if the 302 is beyond salvation or not. I was just curious as to what mods to the engine mounts I would to do to do a swap like that. What mods with have to be to swap in a 351w

The 351w would use the same engine mounts as the 302. The main difference between the 302 and the 351w is the taller deck height of the 351w. Since the block deck is taller the intake manifolds will not interchange between the 302 and 351w because the 351w is slightly wider at the top of the engine. Many other parts such as cylinder heads will interchange.

F100's are a popular truck to hotrod, so there are no shortage of aftermarket kits for motor swaps. If you're swapping motors and intend to keep your current trans you'll have to check bellhousing bolts up. 302 uses the Windsor V8 bolt pattern, meaning a 351W can swap in without changing bellhousing or tranny, the 360 uses the FE Gen 1 and Gen 2 pattern, so it would not bolt up to your stock tranny. I recommend you join any of the Ford truck forums out there, they'll have posts and members who have done these sort of swaps and much more information for you.