My reference materials indicate 1/16-3/32 gap for 1/4" butt welds. I did leave a bevel gap on the frame plates. My initial thoughts were 2-3 times the wire diameter? Just guessing though. My initial plan was to do inside corner welds for most of the internal parts, and then put the outer plates on. The cross-over tubes can't really have internal welds on 3 sides. Hmmm, I can gap those, or spend a few hours beveling the edges I guess. I would probably do that regardless of what weld process I use, just to reduce heat input required for good penetration.
I expect I will need to do lots of stitch style welding, otherwise the HAZ will get pretty large, and distortion may get significant.
I don't think I need 100% penetration in most of the weld seams, but there are several spots where I will need to pay close attention (shock towers, rear LCA anchors).