static drop question

[CMH77]

I'm in process of redoing my '69 lwb stepper. I am not doing a total resto, or any insane mods. Little stuff here and there to personalize, fixing and patching where it's needed. Ultimately, she's still going to do some work, but want to cruise with a little style as well.

My question is about lowering. I have an original trailing arm set up (coil springs at all four corners). For now I'm just going to run the 15's that are on it, but want to look at some slightly bigger steelies or rally style wheels eventually, probably no bigger than 18's.

How low can I go without having to C-notch the frame or making other major suspension/steering geometry modifications? I don't want to do much more than change out the springs and shocks. I was looking at maybe a 3/5 or a 4/6.

The second question, and reason for the long wind up to the first one, is how much am I going to 'undo' the lowering by throwing 18's under there eventually? I don't think it's worth the expense right now to do any less than the 3/5, especially if the later addition of the larger wheels negates most of the drop.

Advice/guidance is most appreciated. Thanks gents.