My new 1969 dodge dart

I just bought a 1969 dodge dart and want to put disc brakes on it...any suggestions..i want to change wheel size to 5 x 4 1/2 - 15" rims...what is the best way to change rear axles.....i want to put in
a 5 or 6 speed tranny...any suggestions.....how much will fuel injection help my gas mileage....any suggestions....thankx