I'm with the other guys on making sure it's actually a Mach 1 first. There's no reason a Mach 1 owner would ever replace the Mach cluster with a typical GT one, period, unless there is something suspect going on.
Also, it might not even be a 4V swap. Someone could easily just put a hood decal on it and a chin spoiler and tell unsuspecting buyers that it's a Mach 1. Alot of people don't even know the 03/04 Mach's exist, yet alone what the difference is.
+1 on pics of the car, I would insist on one of the motor as well.
2007 Mustang GT - 5-Speed, Airaid CAI, 4.10s
2001 Mustang GT "Mock 1" - 5-Speed, 03 Mach 1 Swap, 98 Cobra Intake Cam Swap, BBK CAI & Longtube Headers w/ H-Pipe, 4.10s 302 rwhp / 310 ft-lbs
2015 Ford Fiesta - Wifes
1981-82 Mustang Cobra/GT T-Top
1984 Mustang GT T-Top - Bare Chassis
1984 Mustang GT350 - 302 / 5-Speed
1984 Mustang LX - 302 / 5-Speed