The first picture is the tracks, lined up and clamped to the top of the frame. The top of the frame channel is heavy enough to have been used as the track, but adding the angle adds greater strength and stiffness, if the carriage jumps the track the side angle will direct the carriage back on the track, and lastly it is safer, preventing wandering hands or clothing from getting pulled under the wheels. The more clamps the better, I find for thinner clamping without much depth vise grips work better than C clamps, more precise and easier to install.
Welding and grinding all done , trying out the wheels on the track. Many home built mills use a piece of angle with the 90* backside V facing up, a V cut in the saw arch wheels rides the back of the angle track. This works well and helps to keep sawdust out from under the wheels, jumping the track with those top heavy looking arches, could be disastrous though. In my design the saw arch does not travel, so sawdust is produced at just one point, as opposed to the full length of the mill, so directing and collecting of saw dust should be much easier.
The wheels I am proposing for my mill. They have large heavy bearings, for hoist use, and roll smoothly on the track. When welding the track, it is important not to run full length welds for attaching the track. On the outside surface I used 1" tacks welds, spaced 1' apart, with heavier 1 1/2" welds on the ends. I spaced them out even further apart on the inside. All the clamps stayed on until all welding was complete.
To be continued. Soon I hope, ha, ha. Cheers.