it will slide right in with the original 302 mounts. depending on what tranny you have in the car you may have to change the flywheel/flexplate on the 351, but you can use the stuff from the 302 without any problems since they have the same balance. the only other things to consider are the accessory brackets are not completely interchangeable between the 302 and 351, if you got all the brackets from the 351 use them and you'll be ok. the last thing is the exhaust since the 351 is wider than the 302 you'll have to use the 351 exhaust manifolds or buy 351w/70 stang specific headers also if you use the 351 manifolds and stock exhaust you'll have to modify the h-pipe (or y-pipe, with single exhaust) since the 351 is so much wider than the 302. it's actualy a very easy swap and if you have all the brackets it can be done in a day with the exception of the exhaust work
you may find that the 73 engine has a low compression ratio, compared to earlier 351W versions.
A set of higher compression (read Flat-top) pistons will remedy that, if you plan on rebuilding the engine.
An aftermarket Cam/lifter/spring set will help also.
you probably don't want the stock intake either, or the Carb.
Do you want performance out of it ?
If so what kind.......
A rear end gear ratio is also a good way to improve "off-the-line" acceleration. 3.55's are good choice street gear and still highwayable.
As you can see, you can go crazy if you want.......