Why would my floor board on the drivers side have a puddle of water? I did just replace a heater core. i thought that would solve the problem.

Yes it should be located up under dash on pass. Side prob will have to move a couple things to access

Sounds like a heater core to me, does it smell a little like antifreeze?

your heater core is probably leaking into duct

HEATER CORE AND AC EVAPORATOR DRAIN TUBE STOPPED UP ALLOWING AC CONDENSATING WATER OR SWEAT TO RUN IN FLOOR BOARD.OR LEAKING HEATER CORE WILL CAUSE WATER OR ANTIFREEZE IN FLOOR BOARD.

The miss is more than likely due to old plugs, wires, cap, and rotor. A complete tune-up would probably solve your problem. It's also possible that you could have water in the gas tank as well, because of it sitting so long, or the fuel could be bad. There is some fuel treatment that will not only help the water in the tank but also cleans the fuel injectors as well; I'm not sure of the name, but you can check at any local auto parts store for a bottle of it... As far as the puddle on the passenger side floor goes, that is a pretty good indicator of a heater core with a hole in it. You'll need a new heater core, but before you go replacing it, because they can be quite costly to replace; Have a mechanic look over the entire cooling system, which includes the heater system, because you probibly need new hoses as well and a heater control valve, due to the car sitting for a while, the hoses tend to dry-up and crack, especially if there was no anti-freeze in it- just straight water, that usually will freeze up and rust the heater control valve and rust up the block as well.

heater core and or backed up evaporator drain tube.

your heater core is going out its under the dash on pasinger side of you car you shuold get replaced soon it my lead to over heating problems

leaking heater core or hose that feeds it. look for hoses that pass thru the firewall to passenger compartment. Most times the hose is clamped to a pipe at the firewall so you may just need a hose

Is it antifreeeze from heater core or heater core hose leak?  Is the leak pure water, or maybe something else?
Put it in the garage again overnight with an old white towel in the drivers foot well, best if you can find a way to stretch it from drivers seat forwar to firewall and keep it off of floor mat.  Check it before you go to bed,  then in the morning if no leak found yet.  see if the color/location of the spot helps you identify the source.  I think the heater core is under the dash center.  Anti freeze is green/yellow or orange.
good luck

there are core plugs in the back of the engine block....one is rusted out....you will need to remove the trans to replace them