First off, save the boos,I have a lot of experience with small block Chevys, but am in the dark of small block Fords.  I have a nice 66 Mustang with a tired 289.  I would like to replace the 289 with a 302 and have seen some good advice here, but have not got a handle on the swap from a front sump oil pan to a rear sump engine.  Will the rear sump fit my engine bay?  What are the ins and outs of this swap.  

Well welcome into the Ford community from the land of the evil empire. May the force be with you. Just teasin, hey parts are cheaper over there so we have to be smarter to get horsepower. Sorry no, your rear sump wont work in an early mustang, but the good news is that the oil pan and pump pickup off your 289 will bolt right on to your 302 to make it compatible. No particular problems with the swap because the blocks are the same. You didnt say what year your new 302 was but because of changes over the years there are a few things to look for. Early 289/302 used a 28oz imbalance flywheel/flex plate and front balancer combo while later models used a 50oz imbalance dont mix them up. Later model blocks used a dipstick in the side of the block while early mustangs had it in the front timing chain cover. Front timing chain covers vary and so do the pulleys, early mustangs used 3 bolt pulley to the crank while later models used a four bolt. You can use the heads off your early mustang if you choose(high compression) but you need to pick a rocker combo that will be compatible. Probably more things, what year 302 are you planning on using ?  

I have yet to choose a year model motor.  Not knowing much about what I needed.  I am gathering information on the best choices.  Any advice will be welcomed.  I plan on keeping the 289 in tact and rebuild it at some point to have it available if I should want to go back original.  

It really depends on what your after Dirt. The late model blocks are machined for roller cams/lifters and have the one piece seal which is nice. You can get your flex plate balanced differently and they carry pulleys for the four bolt balancer. The key I think is using the right timing chain cover for what you want to do. Use a late model timing chain cover if your gonna have a serpentine drive and EFI and use an early model if you want to have that stock look with pulleys and fan belts and a boss for a fuel pump driven off the cam. They have some great aluminium heads now so you can get some serious horsepower in either one. Even the late model blocks can be converted to use carbs so shop where you can get a good bargain,,,,and get the timing chain cover and pulleys that you expect to use on your engine.  

Dirt, Welcome to the forum.  I am from the dark side (GM) as well when it comes to the ford engineproduct.  It used to be about the car maker now it's about the "classic" (at least in my view).
jlg