Help with joinery on first solo timber frame project

[Belafonte]

This will be a cart on four 330lb casters for wheeling an earthen oven out from under the porch on a concrete patio. I need two timbers to tie the sills together and probably about 4 to tie the plates together and create a platform for the oven. What you see here has already been built so I need to work with what I have. :-\  It has 2.25 inch tenons laid out snaff snaff from the outside face.  I will put little 1/2 inch oak pegs in these.  I don't think there is enough meat around the post-sill intersection to also mortice another timber in there.  Should I lap joint the timbers or move them inside the posts and do a real mortice and tenon?  What about the platform joists? Cog joints?  The wood is green Southern Yellow pine I milled with my Dolkita 24" alaskan. So far I used the French Scribe layout method I learned this summer.  I attached a sketch up file.  Thanks for the advice.

[Belafonte]

 

  
I just figured out how to export a 2d photo in sketch up

[Dave Shepard]

I'm guessing you took the scribe class at Heartwood this summer? French scribe +schnaff= Dave Carlon. :D

[Belafonte]

I'm guessing you took the scribe class at Heartwood this summer? French scribe +schnaff= Dave Carlon. :D

You nailed it.   ;D

[Dave Shepard]

I took that course in 2009. The week before I took the Cruck course, and in the scribe we took it down and scribed a porch and a bunch of crooked braces into it.

[Belafonte]

Here is how the cart turned out.