i had that leak on my '81, the sealant between the heater&a/c box and the firewall was cracked. i took the lower windshield trim(about 4" wide) off, slapped on a generous dab of black RTV in the crack and that seemed to help. i don't think i got to the whole crack, some parts were hard to access, but i didn't have such a big puddle anymore.
'83 383 with ported Vortec heads, GW upper arms for the 12" discs in front. Custom aluminum dash with full compliment of Auto Meter guages and a little custom interior work.