4cyl to an 8
 

I'm thinkin about buying an 80 mustang with a 4cyl 4spd in it, i was wondering what i would need to do to put a 5.0 into it. thanx

You'd basically need a parts car to get the heavy duty springs, a-arms, struts, spindles, transmission, rear axle, motor mounts, double hump crossmember and exhaust bracket, engine, radiator, brakes, and sway bars (all i can think of off the top of my head). This is all to make it a well balanced mustang like the '87+. Basically all you'd need to get it to be rolling under it's own power is the 2 core radiator, V8 motor mounts, 5.0L and a 5.0L designed transmission (4 cylinder trans won't last against the V8), I don't know if you'd have to change the duraspark II box. The front end may sit lower if you use the stock 4 cyl springs, but many budget drag racers use 4 cylinder springs for better front end lift.
And about the 5.0L engine, you will absolutely need the mustang dual sump oil pan, and oil pump pickup. Good luck, if you can it'd be alot easier to find a V8 mustang for cheap and to use that because in the long run it will most likely be cheaper.

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'84 Mustang 5.0 LX
My car

thanx for the help