PLEASE HELP!!!

I have been looking and lookig with no luck. Heres what im wanting to do...
I have an 89 vert, still 4 lug, i was told that it was converted when i bought it, and stupid me thought it was because of the rims on it WERE 5 lug...
i bought a set of 18x9 2004 cobra rims to put in place of the tacky rims that were on there to begin with and this is what i found...
thare is a spacer plate, not a big problem... went to remove a few allen bolts to find that the 5 lug plat was just covering the original 4 lug...( so what im looking to do is fix this problem by just swapping a rearend? also would like to end up with disc brakes instead of drums... i havent taken the front wheels off yet, but im sure the same was done up there as well... would like to go 5 lug with a larger disc up there as well to really set of these chrome cobras off big time! Pleas help me if you can, this is my first mustang, and i just want to make it what it really needs to be! thanks in advance!

89drop

also wondering if someone can help me out with proper tire sizes, these are 18x9 whats going to be best for front and rear?