stock the '86 mustang 5.0 EFI HO engine came with i believe 200 horsepower and the power you have now greatly depends on how much compression is left. If it has good compression it may have close to 230 NET horsepower mostly because it seems alot of the stuff on that is mismatched. The heads are REALLY restrictive stock and your vehicle has very low back pressure exhaust and intake, but if the engine has some work done to it, it very well could have ALOT more. The only way to say for sure is to run it at a track, get it weighed and then one of the many calculators available (there's one on this site) will give you an indication as to how much power you have....which is alot of work but is one way (besides just getting it dynoed) to figure it out.
-How did you get it so cheap??? Is there anything majorly wrong with it?
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'84 Mustang 5.0 LX
My car