Same thing I did. Didn't matter what cap I put on it wouldn't seal. Then I read somewhere there was an issue with the top of some of the Saleen bottles. So I put an extra O ring on the original cap and it sealed.
I know you've checked it but I keep coming back to the cap. I had a similar problem, the cap tested as good & the system held pressure. The problem was the cap wasn't sealing on the bottle.
It should hold pressure. I'd have the system pressure checked to make sure it's holding the pressure. If there's no where obvious the coolant going I'd pull the lid & make sure your intercooler not leaking again.
The only explanation that makes sense to me with this is a used pulley was put on and no one notice the hand machining on the back. If it was rubbing 281 would of seen evidence of it on the snout.
The reason you want a tighter gap is to avoid spark plug blowout. Under boost the increased cylinder pressure makes it harder for a spark to occur, tightening the gap helps insure a spark and avoid a misfire.
I basically have the Brenspeed Stage 3. Never been dynoed, guess would be 450 whp. Things like the coolant crossover could be hard to find. There's an install manual on the SOEC site that should help, it lists all the pieces you need.
The bracket might be tough. The pump you can find pretty easy. You can use the Bosch pump for the GT 500. As for piecing together, some of the small parts could be hard to find.