1995 V6 To V8 Gauges

I don't know much about the behavior of the gauges when swapped - only commenting here to mention those gauge-faces don't require the removal of the needles so it's a pretty easy change. I don't know if the gauges will then be better-calibrated to the tach / speedo or not. Swapping to a GT cluster won't necessarily correct your speedometer calibration.

For the speedo it would seem the transmission was swapped out, but the rear was left with the v6 ratio? Find out what ratio the rear in the car they were pulled from had, and the rear in your current vehicle has. You can replace the VSS (speedo) gear from your old transmission to the new one and that should fix your calibration. Unless it's off because of tire-size.

You can buy a calibration unit (like this http://www.americanmuscle.com/ford-speedo-tool-9410.html) or swap the VSS gear to the correct one. You need to know how many teeth are on the one now and what the mph % error is. If the speedometer is reading high by 10% and the gear is 20 tooth then you need to change it by 10% or 2 teeth -- to slow it down you would use a 22 tooth gear.

If you want to try to keep the tachometer too, getting an independent read on the RPM (using an external gauge at the coil) can help you determine which of the two gauge configurations is correct. You'll want to stick with the v8 redline for how far you should rev if that was your question.