1. You will need a center sump car oil pan, dipstick, 90 degree oil filter adapter, oil pump pickup, '64-'66 V8 motor mount brackets and insulators, and a '64-'66 V8 center link for the steering. Headers are really pricey and most are fenderwells, so you may be better off finding '64-'66 273 exhaust manifolds. Dakota V8 manifolds work with a little grinding for clearance at the steering coupler. The mounts are available new from Schumacher if you can't find a used set. Be aware that your stock 7 1/4" rear end will not last for long behind a 318.
2.and 3. Don't know for sure about this one. Look at the tailshaft of the transmission. If it looks like a snake that swallowed a pig, it's a 4 speed. If it is a 4 speed, it will work with a bunch of floor mods and reworking of the tranny cross member. If it is a 3 speed the most you will have to do is flatten the seam in the transmission tunnel at the cowl with a BFH.
4. About all you can do to the front end is change to V8 torsion bars and add disc brakes or at the least 10" drums. Either brake option will take care of the weak spot on your current front end since they will use 1/2" lower ball joint to spindle bolts instead of the 7/16" that is there now.
5. You don't have to, but a V8 radiator would be recommended. I ran a 6 cylinder radiator from a '65 Dart with a 318 in a Volare once. You could watch the temp gauge creep up when sitting in traffic or running more than 90 MPH (probably shouldn't have said that).
6. Nope, they are all the same for '64-'66. The cross members are only notched for single exhaust though, so if you want duals you can either live with one pipe hanging down to clear the cross member or notch yours for clearance. That isn't hard to do. There are at least 2 threads on here about that and I'm sure pettybluedart can guide you through it if need be since he did it to his '66 Dart.
Any more questions, sing out. That's what we're here for. :-D