I've now cut the hole in the front bulkhead and carefully positioned the heater where I want it to fit.
I wanted the heater to be relatively easy to remove should the need arise further down the line if there are any issues with matrix etc. so spent quite a lot of time thinking how best to fix and seal to the front bulkhead.
I decided to make up a set of four stainless steel brackets to securely hold it in place and very quickly realised that access to both sides for the fixings was really difficult so opted for several bonded plates with rivnuts in. It took a little while using the tried and tested CAD (cardboard aided design 😊) but eventually all fitted as I hooped. The two lower brackets will be isolated with a foam rubber strip.
The heater box will be sealed to the bulkhead to ensure that no water can get in from the engine bay.
Another thing I wanted to do was to tidy up how the heater hoses connect to the heater unit so I decided to set about making a cover for the heater with the hose connector mounts already in place. Not exactly straight forward but should work well and also look much cleaner when sprayed with body colour.
The heater was carefully taped in position so that the bracket holes could be marked and then drilled.
I made several plates with rivnuts in so that I could access the screws from the engine bay. These plates were then all bonded to the firewall and tunnel top.
Heater fully bolted in place. It will be sealed later when I've finished all the engine bay fitting out.