The '84 rack will fit. The manual rack from any late model Mustang will work. I usually ask for an '89 four-cylinder stripper (no P/S, A/C, manual tranny). You'll need the lower rag joint as well since the splines are smaller on the manual rack. I've got the part number but its at home...any Ford dealership can order it for you. You may need to use the outer tie rod ends off your power rack since the manuals are about 2 inches shorter end-to-end. Some folks will replace both the inner and out TRE to make the rack the same length, but there's plenty of thread engagement when you reuse the power steering TRE's. Just make sure you've got at least 1 diameter's worth of thread engagement and you'll be fine.
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Jeff Chambers
Trophy Stock #3
11.97 Seconds / 114.5 MPH
Chambers Racing Team