Ok did alot of reading now for the newbie questions lol

I've been reading about the rocket stoves for about 2 months and purchased the 4 dvd set and watched them. I love the idea and I want to build a six inch system in my small off grid cabin. Now for the questions. I am wondering if perhaps it might be to warm for my cabin. The cabin is (inside measurements) 13'x27'. It's very well insulated and very air tight. I used 2 inches of hard foam spray foamed around it then put 3 1/2" of fiberglass on top of that plus the drywall. I know the barrel heats up quite well but where I sleep is in the loft close to the ceiling. I will be living in the cabin for hopefully just a year or 2 while i build my house, then turning it into my wood shop. I plan on putting an 18" wide thermal mass 3' high where the living room is, but instead of a bench I would be making it against the wall so after I move out it won't take up much room in the wood shop. I plan on running the 6" duct work out of the barrel then along the bottom of the mass then 90 up vertically and 90 back to where the barrel is and out through the wall just below the ceiling. (there's already a vent there so I wouldn't have to go through the roof.) My questions are:
Would it be too warm to fast to heat up the mass and I'd be walking around in my undies while the mass warmed up lol
Has anyone made the mass out of plain solid concrete with the duct work sealed inside and with stone on the outside (for appearance)
Also I'm looking at getting this and making a form for the feed tube, burn chamber http://www.ebay.com/itm/121581905917?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT
Anyone deal with that at all or should i just stick with perlite and clay?
I'll try to draw up and take pics of the design i'm thinking of going with to give you a better idea. Thank you all for your input.

Hi Jason; Welcome to permies! No, a 6" isn't too big , you just won't be running it that much in a well insulated house. If its too hot while heating the mass ... open a window . Seriously you will hardly need to burn this every other day, you control what time of day the barrel is hottest so don't burn just before bed . As far as a concrete mass , yes , after your horizontal transition area the temperature will have dropped below 400F and you could use poured concrete,... but consider a brick surround, filled with a cob lasagna of cob and rock (no perlite in the mass) Concrete would need to be jackhammered out if you need it gone , cob lasagna comes apart with a pry bar and a shovel and you get to reuse it! Hopefully you would never need to move your mass but..... The refractory you found on ebay will work with a cast core , might look around local for a better price. Fireclay and perlite makes a super hot cast core but it is fragile in the feed tube area. Adding a 50 # sack of castable to f/c perlite would help harden the whole casting. Be sure to isolate your mass from that outside wall however you fill your mass, I used apx 4" of perlite behind the duraroc board to keep my heat inside not outside. Good luck and happy rocketing !