Not to worry. Just mark out the affected area and get busy with the ‘ol cut and paste. You need to go down far enough to provide clearance for changing the alternator/water pump belt.
Shear and fold…
Slice along the lines, push the firewall back, weld in the filler panels.
This is easier if the heater core is out of the way. The throttle pedal bracket will need slight modification as its old anchorage has been altered.
Another view of the same, just because.
Chuck the crank pulley in the lathe, and make some chips. A lighter crank pulley does not seem to hurt anything, though you want to leave enough hub such that the inner and outer sections don’t come unglued over time.
The early H engines used a single groove water pump pulley. Finding one of these makes life easier.