juniormodeler- to "tub" a car is to put fender tubs in it. Fender are tubs are just a bigger wheel well to fit taller or wider tires. to "shve the doors" is to get rid of the door handles. basically sand them off so they don't stick out and mess up the body lines. and to "slam" a car is to lower it so that it is very close to the ground.
corvette_kid- there are a lot of ways to "slam" a car. it would help to know what year Mustang it is, so we know if it has leaf or coil springs in the rear. for leaf springs, you couls cut the spring hangers, add a block between the spring and axle, etc. coils are even easier. just cut a section off of the coil so that its smaller. tubbing the wheel wells is pretty easy. just get a sheet of styrene, cut a section that is as wide as the rear tire and could wrap completely around. heat the styrene VERY carefully, as to not melt it or the tire. lay the warm styrene on the tire, and bend it around. cool it off by holding it under sink tap. then cut the wheel wells out of the kit chassis, fit the tub, and cut it to fit and glue it in. hope this helps.
Jon