Well you've picked the most expensive way to do it.  If you go from four to
five lugs you need new axles in the rear, new disks and spindles up front,
some kind of aftermarket rear disks, brackets & calipers and of course new
rims and tires.  FRPP makes several kits which will let you achieve your
goal but you are looking at a major outlay in cash.( Around $4,000 in parts
is a good estimate.)  A less expensive way would be, live with the current
four lug pattern, (there are any number of aftermarket wheel & tire
combinations that will bolt to the stock set up if you just have to have new
wheels), and just upgrade the rear drums to disk using the FRPP or SSBC rear
disk conversion kit.  Put a 73mm caliper from a Lincoln LSC or Crown
Victoria on the front and your good to go, all a considerably lower cost.  I
have this exact set up on my 88GT, all nice and neat under the stock turbine
rims.  Another good thing about this set up is it uses mostly OEM parts, so
any brake shop can work on it if need be.