on the list, especially since you mentioned brake leak, I would strongly suggest starting there... replace all the lines with nickel- copper and the three hoses, and consider splitting the front/rear using a dual reservoir master... kits are available cheap, line/hoses are cheap... Id go ahead and redo the park brake cables while you are at it, as most cars were automatics and as such, the parking brake cables are usually shot from not being used...
I tend to harp on dual circuit brakes, just because as a kid a neighbor wrapped a beautiful gts dart around a tree after a single point leak left hime with zero brakes...at least dual circuits you will always have at least some braking ability- there was a good reason they were mandated on everything in 66 or 67- aside from one extra fitting, it wont cost but a few dollars more.
handling i agree is not good, ive got a 2000 mercury with the handling package im gonna scrap ou, think I might try those sway bars under the 65...width/weight should be close enough- will see...know the slender front sway bar dont do much, it leans around corners like...well, you know that already
for wheels I opted for 16s- always liked the handling pkg wheels on big cars- our 65/2000/2002 all wear them
please do keep a eye on that frame... hate to be such a naysayer, but man, these frames rot from the inside out in spectacular fashion- and almost all have
i got some parts off one at a colorado boneyard that was beautiful underneath- but bent from rearend damage... looking it over noticed a 'damp' spot on one rail, easily pushed a screwdriver right thru it- looked mint on outside, but inside the eggshell thin suface was a inch of mud/rust...ford had no drains in the lower rails, mud gets in from road splash by the holes in rearwheel wells, builds up a layer of mud from the bottom of rear arches to the front torque boxes...really no way to clean out or even inspect. *IF* I ever got a clean one(doubtful) I would holesaw some holes in the bottom of the rails to flust the dirt out/acid etch/spray paint up in...a hole in the frame might make a weak spot, but in my opinion, any that ever lived outside a desert area are most likely on their way to sproting a hole on their own anyway...you can click the link in my sig to see our 65s rust issues- i was one of the worst messes i've ever seen...
good luck on your 65, this site is a great resource for galaxie fans!