The SN95 is the easiest 351 swap out of all the late model Mustangs. That doesn't mean you aren't in for a lot of surprises. Obvious stuff first. You'll need to take the timing chain cover and water pump off your 302 engine and put it on the 351. You will need a longer serpentine belt; least of your problems. You'll need different motor mounts to lower the engine a little. Use either factory 302 or Trick Flow valve covers. The alternator sits way back on the SN95, and is almost certainly not going to fit with any stock 351 valve cover. You are going to be fighting intake clearance with the stock hood, and the valve covers play a huge part in that. You need a swap distributor for the 351W. Getting your current exhaust to work with the 351 is going to be a nightmare. Plus if you are going through all the hassle of putting a 351 in a Mustang just to put stock exhaust on it, you might as well stick a straw up your ass next time you have to take a dump and report back on how well that works out. You are going to need a 351 swap exhaust.
Give us some more information on what your goals are, and be humble about receiving hard advice. I have been doing this for 26 years now, and I have spent the last 2 days on the phone with Strange and Moser getting humbled myself with how little I know. I keep a consistent message; stick to bolt on parts when possible. I'd hate to see you spend $3k on a 351W junkyard swap that makes 250hp, when you could order a Trick Flow top end kit for $2500 to put on your stock short block that makes 350hp, and installs in 2 afternoons, and a case of PBR.
Kurt