Don't worry.... by the time to get through you'll be able to take it apart and put it back together in your sleep.lol
It always take forever in the first time for everything.

You did use BLUE locite...didn't ya. (BLUE you can take apart again)
RED...you might have to take a drimel to it.lol

If you didn't use any loctite..
mmmm I found like....12 muffulers just beyound the flight line oneday. None matched mine.lmao
And I couldn't even find mine.

you need more, or at least mine did. I thought I could get away with 1/16"...(not).
The engine vibrates when you throttle up or down.

I say at least 1/4 or more to be on the safe side.

Make a gasket from a 12 pack cardborad or something simular.lol

Or just run down to your auto parts store or machine shop they should have gasket material.
Or you can pay the machine shop to mill you an extension.

Or try those DuBro extensions that u cut with a hack saw or drimel ...I guess.