The reason (as far as i know) that valves get burnt when you run close ending exhaust is because when you don't have a certain length of pipe retaining heat within the exhaust, scavanging can actually pull OUT the heat of combustion and the exhaust valves are exposed to extreme heat and then are cooled off really quickly. This causes hair line cracks in the exhaust valves and that leads to pieces of valve chipping off and getting sent out with the exhaust.
By the way, It would be safe to take off your cat-back exhaust (behind the catalytic converters) and it's relatively easy to take out and it will be pretty loud as well and have a nice deep tone and probably help power as well.
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Mustang Parts Specialties ripped me off, it could happen to you as well.
'84 Mustang 5.0 LX
my LX
'89 Mustang GT
my GT