An enlarger is a good way to check your pinhole for hole size and roundness -- just put it in the negative carrier (cardboard and tape usually needed) and crank the head up to project a 10x or so image of the hole.

Pinhole focal length Vs negative size isn't something much talked about but is key to getting good results. The optimum is to use a focal length that is as short as possible while covering the media. Setting the focal length at 50% of the normal focal length is usually the limit. Naturally, at a wide angle the thinness of the pinhole is critical - the metal should come to a razor edge. Emery cloth should be used as the final abrasive to polish off the hole.

The bread truck camera, if it took 3x4" film (guessing at its size) would need a focal length of 2.5" or so; instead it looks as if it may be 6" - more suited to an 8x10 camera.

Using a long focal length is equivalent of blowing up the central 1/6 or so of the negative to make the final print - not a good thing when you are starting out at fuzzy.