It MIGHT be as simple as getting the tach module from a 2.8L Mustang II...
Notice I said it MIGHT...
you'd probably be better off with an aftermarket tach. You can get a good one from any parts store for about $30 (usually a Sunpro Supertach II, these are decent no-frills tachs that are adjustable, and illuminated).
Sunpro, Autogage, Autometer, Fuzion, and Stewart-Warner all make tachometers that are as simple or advanced as you want in several price range.
__________________ 1976 Ford Mustang II Ghia: 302 with a 600cfm Edelbrock carb, Edelbrock Performer 289 intake, Dynomax Blackjack headers, 2.5" exhaust with Flowmaster Super 44s. RJS 11-gallon fuel cell, C4 tranny, chrome 16" pony wheels, fuzzy dice, brown vinyl half-top, and painted in the tackiest color ever (harvest gold, that's why I call it "The Goldenrod").
Also have a 2003 Dodge Ram (lightly modded daily driver/tow rig/office/dining room/home away from home/workshop... I call it "The Big Blue Dawg".) | 
Mustang II tach recalibration 
