Wow this is quite the cool machine, I do like this. I just have a few concerns... please dont take this the wrong way, its just a little safety notation. I really mean this...
I didnt think this was going to be an electric machine. I just didnt read this that good. Not a problem....
The problem I see, is that I see there is some major welding issues. I reckon there is a fluxcore welder being used to construct this. Whenever a fluxcore wire is being used, you need to brush and chip after each time the trigger is pulled, without fail, to make a successful weld. Its like a stick but wire feed.
On your joints you need to have a more consistent bead of weld joining each part, you have several multi point intersections of tube.
I only go into this, And like I said it is cool and its a family project, is that..
The motor of choice is an electric motor, and they have a good amount torque instantly, that is the nature of there design. I just wouldnt want it to rip itself apart while at speed or potentially on a start or while slightly stuck.
There is chains and some flex joints to slightly move to dissipate/absorb the torque, but that will only go so far.
The motor itself I reckon it'll need more secure mounting(I understand your still in construction mode), but it still comes down to the structure itself.
So in conclusion, it is a cool project, not trying to be a debbie downer, but(IMHO) I think you need to look into you welding a bit more, before continuing on.