Here's my recommendation:
any '79-'86 4 cylinder notchback (2 door with trunk) mustang. It's the stiffest structured mustang built and with a 4 cylinder it's unlikely that the engine has cause any stress to the chassis. they're carbureted so a swap to a larger carbureted engine should be a snap. Also the suspension is really soft so there's ALOT of weight transfer stock on those vehicles. A good idea would be to get a stock GT or 5.0L ('87+ with the thicker type) rear sway bar because stock the 4 bangers don't have one. The crossmember is the same, you'd basically just need the drivetrain, 2 core or larger radiator, motor mounts and a good idea would be an 8.8" rearend from a later model as well. Brakes on the 4 cylinders is the same as the V8 during the early 80's so that's not really an issue of comparison. It would be a VERY good idea to update to later model brakes ('87+) which would require i think the master cylinder, spindles, struts (because the mounting points are different), lines and the obvious things like rotors, pads, etc. Good luck, keep us posted on what you're looking to do and what you've done so far!
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previous name: 84stangLX
'84 Mustang 5.0 LX
my LX
'89 Mustang GT
my GT