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Your heater core is leaking it looks like.
As for the floor pans, just take your time and make
sure you grind everything flat and clean.
Sometimes you have to trim the new ones slightly because of
too much overhang between panels.
And spray it all with etching primer AND rustoleum before you
recoat it with floor coating. Rubberized undercoating works for that also.
I just restored my 79 notch.
Good luck man !

The heater cores are supposed to have a flow restrictor in the input to reduce the pressure at high RPM.



See http://home.netcom.com/~skent/mustang/1990/heater_core/hcore.html for more information

Looks like a heater core leak to me. Mine just started leaking this past week and this car is really starting to irritate me.

I had to replace the tranny crossover earlier this year due to rust. Right now I'm putting a new gas tank and lines in it due to rust and now the freakin heater sore is leaking. Oh, and while I was fixing the crossmember I noticed oil leaking from the tailhousing where the driveshaft goes in. I reached up and pushed and pulled on the D/S and you know it, the bearing is loose. I also have a brand new oil pan sitting here due to flaking rust on the original pan. I wanted to put a tubular front end on it but that is going to have to wait a while longer now since the $450 gas tank repair came up. Am I rambling.... stop me please....