Thank you everyone . . .
i live in a country where car parts are very hard to get especially for Mustangs, and buying that bar will cost me over $500 to be shipped and taxed withoit the cost of installation. . .
i m very scared that one of my wheels will fly off on the high way. . . .
i think i should just go and buy a set of larger wheels, but here stands another issue which is performance . . .
if i swap one of the spacers by a thiner one, would it cause any problems ?
You're welcome . . . Oh I understand now -- the local cut-and-weld job might be less costly than ordering the new part from the US; but I would still hesitate to do that, since it is permanent and cannot be reversed.
I have not used spacers, but I'm pretty sure the thick spacers actually have extended studs in the spacer, and the spacer bolts on to the hub; if it is done right there should not be a problem, but I am not sure what they did. You "should" be able to use your original lug nuts, I am not sure why they changed them, if they are steel and they are not stripped, and the studs are long enough to get good engagement, then there should not be a big problem. But in any case, because of all this, spacers really aren't the best thing.
But if you are using spacers, a thinner one is better than a thicker one, so I don't see any bigger problem with using the 1" spacer on one wheel.
Wider tires won't reduce performance much if any. They might reduce your gas mileage just a little, might not be noticeable. If you are concerned about that, go with light-weight 18" wheels, Forgestar is a well known brand here in the states that makes fairly light wheels at reasonable prices. If the "offset" of the wheels is correct, it will make the tires fill the fenders like you wanted.