As someone mentioned earlier, your getting through this very quickly.
Firstly you need to take the head back off, 1) to anneal the head gasket, that thing will never seal unless you heat it to cherry red all the way round, you can then quench it to help remove the carbon deposit.
2) more importantly.... that’s this turd at the top of the cylinder?
It looks like something is breaking away, this needs investigating and sorting because if it gets worse you won’t have a Nice cylinder for very long.
The next one is the Siamese pipe, there aren’t any decent ones on the market unless you stumble across an original one, I have an original and I made a copy from that. However that’s not possible for a lot of people, the main problem with them is this:
The right hand down pipe hasn’t been cleared properly before the mating pipe has been brazed on. What happened with mine was the gas couldn’t escape properly and it buggered the left hand cylinder. The picture was taken after I had started dremelling all the excess metal away so it was worse than that, by the time I had finished there was no lip, but gas flow on these pipes is still poor compared to an original pipe.
Don’t know if you know or not but you’ll also need some 5/16 rod (or 8mm) to make a tie rod that fits through the tubes at the headers. The problem with the left hand pipe and why it won’t go in properly is because there is too much curve on that left hand pipe. There isn’t enough straight section. Seal it in with RTV sealing and tap it in as far as it will go then put the tie rod in.... you’ve also forgot to put an exhaust clamp on where the two pipes meet.