(Not Quite what I had in mind..!)
I took an hour or so to pluck up the courage to dismantle the heater to see what was inside. On reflection, nothing too dramatic was inside so it was on with the show... On the side panels, I Dremelled 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 2.5 cm smaller than the heater. The total length of the heater is 26 cm and the available horizontal space being ~23.5 cm... 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 of 26 cm
This vertical configuration leaves 30 cm for the battery and battery tray, which is plenty
Note... There are three speed outlet (wires) on the motor, but only two are usually hooked up via the GD loom. I've enabled the third by running a separate control wire to a three position fan switch. The existing loom takes care of the other [common] connections... More on this is in the 'Electrics - Loom Dashboard' article.
Drilled out the two holes for the air ducts
As you can see, they are not the neatest of holes, and this was because when I drilled them and trial fitted the box I realised I could create some more room for the heater pipes so I whittled away to create as much room (~5 cm) as possible 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. 5 cm is plenty of room to connect up and get in to tighten the pipe clips without too much difficulty, so I've got room to spare.
Footwell Air Vents
Fitted the footwell heater outlet vents to the dash shelves. However, I had to enlarge the holes a smidgen. Also 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.
Disassembled vent and the pre-drilled hole
The notches in the aluminium tray holes were ~1.0 mm too small for the plastic lugs on the vents, so I enlarged them slightly with a square file... A small but significant modification.
I also trimmed the footwell vents outlets as they were overlapping the lower lip of the dashboard. Trimmed off the excess from the outer plastic rim and cut a ½ cm insert in the dashboard lip to achieve a flush and firm fit. i.e. with the leading edge of the tray being tight up to the inner trailing edge of the dashboard lower lip.
Windscreen Air Vents
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 the vagaries of GRP and temperature variation, so it's not a problem. However, I know it is there and it will bug the hell out of me; so I will use the centre of the windscreen as my 'visual' centre reference point. 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 dead-centre) and marked for the innermost vent screw holes. I also ensured each vent was 8 cm "in" from the top lip of the dashboard i.e. the scuttle top curve. The 8 cm is not quite what it traditionally should be but I think it looks in proportion. 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 to be routing out. But first I re-verified 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.
Demisters (Air Vents) & Footwell outlets
The two outlets from the heater, feed the two footwell and the two demister air vents via a spitter.