[yoh!!!]

Hi,

my 74 has a AC unit and the unfortunate holes ...
... the plural here indicates that there are more (check wheel wells on the passenger side, engine sheet metall around the oil cooler)
I would prefere the car would not have the AC unit, but this is how it came. I do not think that the structure of the unibody is impaired by cuting the hole for the AC - at least not for driving it - it might be a different deal crashing the car.
Welding... sure you could weld them shut with a MIG welder. If you do the job try to find a donor car to cut out a sheet metal pice rather than a flat piece of metal to close it back up.
An other option would be to score a under dash and trunk unit and install an AC in the car, that would cover the holes nicely. AC pices are occationally offered on ebay.

Patrick Koch