all 4 of my g-forces arrived already from tire rack, and the two front wheels. The rear two are custom made and will be another few weeks, but I will have the fronts mounted this week to see how they fit. I'm not going much bigger up front than is on there now and I have all kinds of room so I dont think the new fronts will be tight.
I used the tire calculator to compare my existing with my new setup and that jpeg file is on my work laptop which I'll post a copy of later. I researched the fit for hours and I'm confident what I ordered will fit. The important thing is the space the wheel and tire occupies, and if your 18" tire fits within the overall profile as mine it should work. That calculator is a great tool.
Oh, for the rear, I removed BOTH rear wheels and measured the distance from the mounting surface of the drum brakes (as I assume is the case with most) to the closest point of the leafspring about where it would contact the inner sidewall of the tire, and I came up with 5-3/8" on the right and 5-5/8" on the left. The fact that the two sides will probably vary is something you need to consider as well if you're going to be pushing it.
Sorry if this is slightly off topic but I think it applies for the rear anyways.