My haynes manual says you need to take bottom part of dash out, the gearstick surround bit.

4 bolts hold heater box under dash, but you need to remove two screws from engine bay side where the pipes come through bulkhead and remove that plate and 'gasket' type thing before you undo the bolts.

my car is stripped to shell and only thing left was heater box so very easy for me to get at everything.

you could do it in a couple hours, but i'd take passenger seat out to give you lots of room.