Managed to get a O ring from the brake joint for free. Shout out to Better Brakes in Bayswater. They also said they're happy to flare any hard pipe i need to run for the project if I needed help, I didn't even ask they just offered when I asked how much hard pipe was per roll. 
 
No joy on the bolts though, two specialist shops couldn't help me so I bit the bullet and grabbed the grinder, turned out ok, might end up masking things up and painting the bolt heads now.

looks good.
I would have angle grinded the bolts down too.
Was it the pesky orings that go on each side of the spacer?
 
I had the same issue when one of the caliper pistons got stuck in on the 4 door (still cant get it out), and needed to swap the whole caliper housing over for a spare.
Interesting the vented disk that had been used on mine was a DBA part number, that matched to a Mazda 323
 

It was the spacer O ring yes, torn on delivery, I let the RS shop know in case it was a batch issue.

Mazda 323 would make sense actually, I wonder how much a set of those discs go for these days . Might be worth grabbing a set and doing a test fit if they're cheap!

More stuff happening, went and got a battery tray and box so I could dummy fit it ahead of paint/panel work. Decided to go with the drivers wheel well to offset the weight from the spare.
 
Also tried my hand with filling up some smaller holes around the car, did not too badly but it brought into focus my need for a filing sander / die grinder. I have nothing that can reach to smooth the welds on the back/under side.
 
 

 
Now I just need to decided if ill use the box or not, I think the box does make it look cleaner. Next job is to make up a MDF panel to close off the back end of the boot with little L bracket mounts. I'll use a cardboard template to get the shape then get the jigsaw out.

Nice progress, the O rings are a pain, they either break or are dysfunctional useless..
When I did my yellow van brakes I made my own spacers from 6061 ally and of a wood spade bit to create the step for the O ring. Bolts were like yours, 1 longer than needed but there wasn't any option so I left it. Rotors were BMW E36 I think and needed some lathe work to get to fit and so did the spindle, I highly don't recommend it !
Just thinking after the fact with your battery tray, I would sit it down in the cavity and board up that side, it'll give a bit more space.. if that matters..

I thought about having it down in the well and that was the original plan, I changed my mind though because I couldn't figure out a way to anchor it to something solid. The three bolts on the inside go through the floor and the edge of the rail so its a good 3mm of material. 

It probably would have been fine on the thinner stuff but cowardice got the better of me  

If you run a dry cell it doesn't need covering and venting like acid batteries do. The battery in my van is laid flat under the drivers rear compartment, opposite the spare wheel pocket.

Mark, This was from one of my old cars. I had the local sheetmetal shop bend up the stainless and as you can see its a typical old lead acid battery (this was 20 years ago).
So from memory I went through the side of the box section around where the hole is to get to the leaf hanger bolt.
And underneath I would have supported it from the bottom of the well.
Never had a problem

Hmmmm that does looks good. I saw your post as I was standing in the MDF aisle looking at boards for hoarding the boot off, completely made me change my mind and walk away . I may have to enquire around a stainless box then, I have a lot of fab places near me so hopefully the pricing wont blow my head off!

If not stainless, do it in alloy 😊