There is nothing that specifically limits you from
running a V8 in a 4 cyl car without any changes. However, you will
likely be riding in an unsafe car and breaking the rear end eventually
The chassis is identical, with a few exceptions: The brakes, the drivetrain, and the shocks/springs. I will cover all three.
The rear end mounting points for the 8.8 and the 7.5 rear end are identical. As a matter of fact, the axles, drums, and all brake hardware are interchangable. The only thing you need is the axle housing and the differential. It is much simpler though, to just buy an 8.8 out of a junked car and install that.
The front suspension is a little more difficult to change over, especially since noone I asked was sure of what interchanged and what doesn't. Here is what is the same: The tie rod ends and the lower ball joints are identical (even though the parts stores list two part numbers for the tie rod ends and charge more for the V8 one). I called TRW and they confirmed that their tie rod ends are packaged with the same production number, but that parts stores label them differently to get more money for the V8 end. Anyhow, if you want to install GT brakes, you need new: rotors, bearings and seals, spindles, calipers, pads, brake distribution block, and master cylinder. You do NOT need to change: tie rod ends, ball joints, sway bar (although the V8 Mustangs have a fatter bar) or anything else. The front strut mounting pad on the V8 is skinnier, and a spacer plate available from Monroe is needed to use your 4 cylinder struts. However, since you have the front end apart, it is in your best interest to chage the struts/springs since you have them accessible. If you are changing to 5 lug rotors instead of 4 lug, all parts are identical, except that you should use 1986 SVO rotors instead.
The tachometer from a 4 cylinder will read double the actual RPM's if you use it with a V8. The older style dashes (79-86) had a settable tach. This is no longer the case. You will need a tach out of an 87-93 car. While you're at it, grab the 120 MPH speedometer too. NOTE: 87-89 and 90-93 used differently wired dashes. Unless you are good at rerunning wires (it can be done), make sure you get one from the correct era.
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