Hey Max,
Just leave it now until tomorrow & it cools down ! Dave said you can't break a 4K, but you've got a serious problem there. if I was in Sydney, rather than Brissy, I'd be round there in a flash. Love to know what's causing this issue.
While the engine is running & this exhaust manifold is glowing red hot, does the engine make any unusual noises ?
I think it's back to the drawing board. Forget about carby & dizzy tune & settings. Time to pull all the spark plugs out, do a compression test, and maybe a leak down test, with compressed air. My guess is that big bang you experienced has blown something. I think there is something basically wrong with the engine, and you need to check out the engine integrity first before anything else.
Cheers Banjo