On mine I found that it was 18 ga and 16 ga, or 0.047" to 0.06". Since you are doing this, you should consider doing the 26R mods, relatively simple and makes the chassis much more rigid. I think I have 3D CAD drawings of the parts and can send you 2D PDF's to trace and cut out the parts if you like. They are all flat pieces, so should be easy to make. Let me know, Thanks Allan
Note: Just realized that this was the Europa Forum and not LotusElan.net opps, never mind.