fwiw, my front "outrigger" frame ribs (which I'm thinking, are what you're talking about) don't go out to the C-Channel. They stop short, and DIDN'T have anything "scabbed" on to reach out to the C-Channel. Maybe the person building yours actually went "above and beyond" . . . I re-built mine the way they originally built it - so no connection out there at the corners - just the plywood/channel/skin connection - which I'm thinking is sufficient.
And also, fwiw, I used 3003 for my belly (I think that's what it was originally). Make sure (just my opinion) that you don't get a tempered alloy, as the 4 corners are pretty (as in "very") difficult to "configure".
Love to see some photos . . . 1960
was the best year (wink wink).
Oh, and in case you haven't read elsewhere in the forums, the original axle spindles on Caravels have some failure history you might want to "read up" on (and maybe the "vintage" of that axle issue is different than yours(?)).