[DampRobot]

To be clear, it sounds like you are only planning on supporting the bearing blocks on one side, and to leave them above the frame inside C-channel. I really wouldn't recommend this.

For starters, there's going to be a lot of stress on the bolts (I presume you're planning on using bolts) that connect the wheel blocks to the frame. If the robot weight is equally distributed over the 4 wheels in contact with the ground, then there will be approximately 37lbs/wheel. Assuming 1 inch by X inch C-Channel and that the wheels are 1 1/2" from the frame and two bolts per wheel truck, then there will be at least 37*(1 1/2)/(1/2)*1/2=55lbs per bolt. In my opinion, this has a lot of potential to either damage the tapped hole in the wheel truck or to bend the C-channel. It would be best to make the wheel block a tight fit in the C-Channel.

However, the internal dimensions of C-Channel has much lower tolerances than the outside, which would make it difficult to machine a slip fit wheel truck even with CNC machinery. Also, there is usually a rather large radius inside of the channel.

Before you decide to just bolt some of the bearing blocks that bear the whole weight of the hole to some tubing, think about the stresses and tolerances involved. You may have thought these things through, but as I see it there is a little advantages of this system over a more conventional wheel block system.