The tachometers on Mustang IIs are kinda finicky at their age, I've got two Mustang IIs right now and neither tach works, there is a plastic circuit board on the back of the gauge cluster that and a wiring harness you can check though inside the dash.
Fuel sending unit may or may not be available from parts stores, I haven't looked because the one in the II I actually drive works, so I couldn't tell you whether it is or not.
The temp sending unit is a standard piece across much of the Ford line from the 70s, readily available at any parts store.
As far as exhaust, your best bet would be a competent muffler shop, as prefab systems for Mustang IIs pretty much don't exist, but you can make a simple passener-side exit system with a hanger, a 90-degree bend, a hanger hung from the front of the leaf spring perch (will exit in front of the right-rear tire) a handful of exhaust pipe clamps and a glasspack muffler if the remaining pipe comes down far enough at home... of course, it'll be loud as hell!

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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".)