The 3.8L does have a reputation for blowing head gaskets and the 3.0L is a really solid engine. I'm sure one could go 250,000 miles, but don't think it won't cost anything. There's routine maintenance, how you drive, and how it's been maintained that makes a huge difference on how long it lives. Plus there's normal wear and tire items as well. My parents had a 3.0L taurus sedan that went 225,000 before they sold it and they are pretty dependable, economical forms of transportation. I don't know how reliable the SHO (yamaha made) engine is, but it's probably good because i've never heard anything bad about it from anyone.
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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