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Mike's Dart said

Let me pick your brain on this; We've had moderate rain a couple times the past three weeks and no leaks in the front floorboard. Had a really hard rain last week and I found a pretty healthy puddle in the passenger side floorboard. It has rained couple of times since and no leaks. I found it strange the leak occurred only with the heavy down poor? I ask because you really seem to know these old cars! Thanks!

Mike

Click to expand...

Under those circumstances I would be VERY suspicious of it being a cowl overflow down into the vent box.
Ever clean the drains out?
They are between the ends of the cowl and the fender, and if those get plugged a hard rain can overflow the inlet for the vent and end up on the passengers floor.
The drains are just a couple of openings where the two pieces of the cowl come together.
If you look close, does it look like the inside of the vent box has had water in it?

This is what one person did to get the collected junk out of each side, but the factory drains are where the arrow is pointing.