Paultvx, offset and backspacing are different but related. In any case, to move the tires farther out, you want a SMALLER offset/backspace, so the issues I brought up are mostly irrelevant to you. Cragar's pdf catalog has a nice diagram showing what all these dimensions really mean. Here's how to convert between offset and backspace (units in inches):

BS = (W + 1) / 2 + OFFSET

OFFSET = BS - [(W + 1) / 2]

Note that (W + 1) is the industry standard method to express the external width of the rim measured lip-to-lip. The "official" rim width W is just the internal width the tire sees. Also, note that OFFSET can be either positive or negative; negative offset wheels stick out more.
-- Quest