E7TE 5.0 windsor heads take a NGK UR45. If you have a 1988 5.0 mustang with stock heads this is the plug you need. The TR6 has a longer shank and will thread too far into the combustion chamber on a E7TE equipped 5.0. Using a TR series plug will probably cause engine damage! If you want to go to a colder plug with stock heads, go either 1/2 a heat range colder with the UR5, or go 1.5 heat ranges colder and use a UR6.
Go to this website for more info on heat ranges and NGK plugs:
2.3 turbo takes a TR5 or TR6
High swirl 1986 GT & other 5.0 passenger car take a TR5
GT40 heads take a TR5
Most SOHC & DOHC Modulars take a TR5
5.4 Lightning takes a TR6
TFS twisted wedge takes a FR5
AFR 165, 185 take a FR5
I run TR5's in my SVO & they are a direct fit. I do not yet need the TR6's but I might when my big turbo gets here!