Hi and welcome to diytube ...
about the name ... the original 1626 amplifier was called "The Darling" so this one was dubbed "Clementine" as in Oh My Darling Clementine. Paying homage to an old and funny Bobby Darin song called Clementine about a 299 pound chubby chick we called the one with "fat" tubes a "Chubby Clementine" https://www.youtube.com/watch?v=0UosSbCXTUw
You can use pretty much any 6L6 based tube with no changes, 6550, 5881, KT-66-77-88, EL-34 etc. Don't bother with KT-299's or anything extravagant, this is a low power amp !
I've built quite a few of these, and find the generic Russian tubes on eBay to be perfect at a low price.
a 100K volume control and A/B input switch is easy, there is plenty of info and pictures in the forums, use the search box
The screw terminals make it an easy build. The board can be removed easily. Oh - the power transformer center tap wire IS soldered.
My suggested steps:
1) before soldering anything to the board, use it as a template and mark the chassis with a Sharpie through the holes, including tube centers
and use your transformers as templates as well, drill holes for power connector, speaker terminals, RCA jacks, volume control, input switch
2) do the metalwork. tube sockets need 1 1/16" hole.
3) mount the parts
4) assemble the board and put it in place
5) connect the wires
6) listen to the results
Plenty of help here on the forum, lots of pictures on my web page at http://tmamps.com