[mrein3]

The firewall sheet metal is easier to modify than you think.

Refer to the picture Hutch posted.

The non-existant yellow hole has a factory dimple to line it up for the heater core. Use it to center your hole punch. The red and yellow holes in the picture that look like they don't line up really do.

Take a piece of paper to your non-ac firewall and make a trace of the heater hole and the two holes for the heater core. The heater core holes become your reference points. pic 1

Then take your future AC evaporator holder assembly and make a trace of that, including the two holes for the heater core. pic 2

Then take the two tracings and line them up using the heater core holes to line them up.

Cut out the piece of paper you need to create the metal you need to weld in. It sort of looks like a "J". pic 3

Finally hold your metal J in place and make any final trimming adjustments. pic 4

Then weld in the J and cut out the hole using your template from pic 2 for where to cut.

Pictures to follow...

pic 1 -