I might suggest to discover the cause of the "no drain" issue first. Then you can braze-in some metal on the cowl area and close up that "drain" of sorts.
There was a thread on another forum about cleaning out the rocker panel drains and then coating the innards with a sealant material. I suggested an oil-based compound that would wick into the nicks and crannies of the structure and also be absorbed into any remaining debris, rather than an encapsulator compound which would seal it all.
A friend had a '70 'Cuda HEMI that he had to fix some rust issues in the cowl area, years ago. Not sure where it was on the cowl structure, but it seemed like it was toward the bottom on the rh side?
The cowl area is supposed to be one of the stiffest areas on the car body itself, from what I read years ago. FWIW.
Looks like you are making good progress toward a nice cruising season!
CBODY67