On a '68 I put a mini-tach where the clock goes and replaced the ammeter with a voltmeter. You can definitely get the voltmeter from rocketman. I think he also sells the tachs as well. Here's his website.
Its hard to beat the look of the '65 GT-350 setup but it will not work with a '66 padded dash, Somebody needs to make a '66 padded dash with a build in GT-350 gauge pod. My '66 GT-350 just has the tach screwed to the top of the dash. I wish they would have done something like they did on the 1965s.
1965-1966 Dash Grill w/Gauge Pods in Vinyl Color: Black Why not make your classic Mustang look like a new Mustang? This aftermarket Vinyl-covered ABS gauge cluster goes on top of the dash between the defroster openings. It comes with two vents (one on each side) to allow for ventilation of...
Pictured is my solution to Tachometer fitment on my 66. I can still read my speed because the end of the Speedometer needle shows behind then column mounted Tach. Granted, I can't see the odometer but I don't care about that.