Wheel fitment with e body rear

An E-body rear is 61-5/8" drum to drum, a 68-70 B rear is 60-1/8". So the E-body is 1.5" wider which means an extra 3/4" backspace on each wheel to keep the wheel in the same place. My 18x10's are pretty much centered in the wheel house on a 68-70 B rear and a 1/2" spring offset with 7" of backspace. So if they were going to stay in the same spot in the wheel house with an E-body rear they'd need 7.75" of backspace.

Those FR500's only have 6.4" of backspace, which isn't enough for even a 68-70 B rear.