Twain is using very strong regional slang. The phrase occurs in the context of a dead miner's friend attempting to persuade a clergyman to give an oratory at the funeral. The slang is deliberately being exaggerated for comic effect, in order to emphasize the fact that the clergyman cannot understand the miner's dialect.
"Toot your horn" in this sense is an analogy meaning "go all out!", "don't hold back!" or "be as over-enthusiastic as you like!" When someone is celebrating in an exuberant way, they might drive around sounding ('tooting') the horn of their car.
The phrase "if you don't sell a clam" is quite difficult for a non-native speaker to understand here. There is a US slang expression "I didn't sell a clam", which means "I didn't sell anything at all (not even a clam, which is almost worthless)". But the speaker does not mean this - he is leveraging this common expression to avoid blaspheming. What he really means is "as if you don't give a damn", but this would be an inappropriate phrase to use before a man of God, so he substitutes another phrase which rhymes instead.
So the whole phrase actually means "be as exuberant as you can (in praising the life of the dead miner) - go all out as though you don't give a damn."
"We've got a shebang fixed up for you to stand behind, in No.1's house,..."
A 'shebang' is an all-purpose slang word meaning, more or less, the 'thing'; that is, whatever we are talking about at the time. It is most often used in the phrase 'the whole shebang' ('everything', 'the whole works'). 'No. 1' means God, and so "No.1's house" is a church. He is trying to say "We've arranged a pulpit for you to stand behind in the church,...", but he doesn't know the word for 'pulpit', so he substitutes 'shebang'. In modern slang, it would probably be "We've got a thingy fixed up for you..."
Thank you so very much.
It really helps alot.
And to think that I translated No 1's as Buck Fanshaw.
It makes absolute sense now.
I can't thank you enough.
I am trying to translate the story, but due to the lack of resources, I need to come up with the meaning on my own.