Hi Wayne, keep the frames the way you have them as adding the planking will give you a greater displacement and this is a good thing.
I don't like balsa for this job, you do not need the lightness so go for a harder wood. You will see the frames through thin balsa as they simply use the frames as corners.
Ply at worst cut with grain running along hull length, but it works, any knot free straight grained wood.
I would use 3 mm planking to allow for some smoothing down afterwards. Planks can be quite expensive.
I have used 3 mm liteply ( cut 5mm and 8mm wide) lengthwise on one yacht smoothed it to nice fair curves and then 1 mm x 6mm wide diagonal planking. If you cut with a circular or band saw allow for the cutting losses.
Very strong hull this way and with diagonal planking there is very little wastage. But the planking is not cheap, work out the area and add 10 %, you don't have to have long planks so they could be cut from Bass wood or obechi.
You can widen the keel simply by double planking.
I think construction in this way will give you the displacement to carry the extra lead as low down as possible.
Have a think about how you will bolt the lead through to the hull to make it a good mechanical joint.
Are you stepping the mast on the keel or the deck?
I think it should all work for you.
regards Roy