Are your tires allowed to stick out of the fender well? A 20x10 w/ -24 is going to stick out about 1-2inches in the front an about 3/4 to 1 1/2" in the back. If you are not legally allowed to have your tires stick out or don't want them too, you are better off with a 20x9 and -12 backset on an older body style truck. the front ends on our trucks are about 3 inches wider than the rear, a 305 on a 20x9 with -12 may just slightly stick out on the front but will tuck just under the edge in the rear
I have 20*9 with a +14 offset with wide tires and they stick out some so if you go all the way to -24 your going to stick out a ton