Well sounds exciting!!
First, before even writing one sentence, I have made it a habit when "assisting and suggesting" possible timber framing scenarios that involve clear needs of PE expertise...to suggest just that...GET A PE with timber framing background...
Sorry, that is just a disclaimer that I have be well advised to give, in as such that if one has to ask the questions..."is this big enough"...then they either need a PE and/or can't properly do the engineering, timber, and joinery assessment themselves.
Please know I am very excited for you and wish you all the best, yet our share "opinions" are just that..."opinions" and not the replacement of good knowledge and experience and/or the help of a PE...
With that stated...here is my two cents:
As a facilitator of traditional historical and natural builds...concrete of any kind is the last thing I like seeing on a job (unless a natural concrete of some form is applicable for a historical restoration) so I am pleased you are going with a raised floor and a stem wall.
From what I have gleaned thus far, you need the floor of an old Mill or Barn...not a garage or house. Having restored, blueprinted and seen thousands of such structures over the decades, I think a 6x6 is barely the minimum...
I would also suggest that a "fully housed dovetail" is going to be the only form of this joint I would even possibly consider as any other "dovetail" shall be inadequate and present with possible shearing issues especially over time and in White Pine.
So with that said, your loads are going to be "transient point loads" of "high weight amounts"...very similar to a barn, mill, and/or industrial garage or other such structure.
The planking on top should be a minimum of 50mm (2") to 80mm (3.15") splined or T&G and/or toggled at 300mm (11.81") intervals. Plywood can be part of this floor diaphragm matrix, but is neither necessary nor traditional.
If staying with 150mm x 150mm (~6"x6") then my rough math suggest a spacing of 400mm (15.75") to 500mm (19.69") minimum for the potential loads this floor could (and probably will be) subjected to.
The Joist could also just rest on top of the sills with added height to the posts to compensate for elevation gain.
Spacing could go to 600mm (23.62") and maybe a bit further if a thicker plank floor matrix is employed.
Many (most?) traditional structures of this nature (i.e. barns, and older mills) only used log bolt sections as joists, hewed on two side laid on the flat...no joint often...resting on sills...Their average spacing is 400mm (15.75") to 600mm (23.69") with most being 175mm (6.89") to 250mm (9.84") thick with a 50mm (2") plank floor on top...This is just a mean average range I have observed...
Good Luck and post pictures as you progress...
Thanks for sharing the project,