(Not Quite what I have in mind..!)
I took an hour or so to pluck up courage to dismantle the heater to see what was inside. On reflection, nothing too dramatic so on with the show... On the side panels, I Dremeled off the metal wings as described in the GD documentation and Hammerited the new bright edges to protect the bare metal from rust.
Measured the cavity to get my "eye-in" with what I had to work with.
Made a cardboard template of the heater back-plate to define where to drill the two air-hole ducts. And taped it in place. But, when test fitting the actual heater unit (with the air outlet manifold taken off,) I had an issue as the heater would not fit horizontally as it was supposed to. This was because the horizontal space within the under-wing bulk-head compartment was 2.5cm smaller than the heater. The total length of the heater being 26cm and the available horizontal space being ~23.5cm... Not good. Luckily the heater has a blanking plate to allow for the blower motor to be mounted vertically.... Good.
Bottom air outlet where the fan will be (underneath the radiator box unit
Heater total length 26cm
(This vertical configuration leaves 30cm for the battery and battery framework, which is plenty)
Drilled out the two holes for the air ducts -which was a nightmare. GRP dust everywhere. I'm really glad I did this outside on the driveway with my goggles and mask on.
As you can see, they are not the neatest of holes, and these will be the un-neatest and most inaccurate holes on the whole car. This was because when I drilled them I realised I could create some more room where the pipes go, so I whittled away to create as much room (~5cm) to the left of the heater box for the radiator inlet and outlet pipes. Reading other builders' blogs, it seems this can be a very snug area for the pipes. 5cm being plenty of room to connect up and get in to tighten the pipes clips without too much difficulty. With respect to my drilling skills, all my other hole drilling will be spot on.... Trust me.
Fitted the footwell heater outlets vents to the dash shelves. However, I had to enlarge the holes a smidgen though. When separating the plastic outlets I took great care to not break the plastic lugs on the outlets when separating them. This was quite fiddly.
I also had to enlarge the locator notches. A small but significant modification.
Windscreen Air Vents
The demister vent slots are next on my list. Checked the pre-marked marking, and one was off by 4 cm... Yep! That's a huge 4 cm... Not good!
Did 'my' measuring, and discovered the centre of the screen with respect to the centre of the scuttle was off by 3 mm. Not a huge amount and I dare say, way within tolerances of flexible GRP and temperature variation, so it's not a build problem. However, I know it is there and it will bug the hell out of me; so I will use the centre of the screen as my 'visual' centre reference point as I want the vents to be optically correct and symmetrical with respect to the driver and passenger's perspective.
Using the mirror base-plate as my centre reference point, I measured 18 cm to each side (of centre) and marked for the innermost vent screw holes. I also ensured each vent was 8 cm in from the top lip of the dash' i.e. the scuttle top curve. The 8 cm is not quite what it traditionally should be but I think it looks better. Also, I've ensured the vents are parallel to the scuttle horizontal.
Drilled the fixing holes and temporarily bolted the escutcheons in place. I then scribed the slots that needed routing out. But first I reverified the positioning with the Dremel straight line cutter, and then did the deed..... Traumatic to say the least. However, I am very pleased with the result and the escutcheons now look like a fine pair of silver eyebrows.