If there is a groove on the inside of the steady plate it is probably due to the steady plate being bent slightly. This can occur if the spindles are not all at the same height. You may have a spindle or spindles pulling the plate down on one end and causing the groove from the ET157. I put a bit of effort into my timing chests to get the spindles all the same height. It is not complicated, but it takes some thought and a lot of shims from McMaster Carr.

I don't remember all the small details from memory, but I believe that there are two groupings of spindles: one group has the ET98/1 shim and the other uses the ET173 shim. Each spindle has one or the other shim between the steady plate and the spindle. If you note that the spindles all have 5/16" threaded ends to bolt down the steady plate, then any shims that have 5/16" holes are meant to sit on the shoulder of the spindle and are captured between the steady plate and the spindle. Because the two shims have different thicknesses, their respective spindles need to be set at different heights.

When I say that the spindle has to be at a different height, what I mean is that I use the timing case cover mating surface as the datum for the height. The spindles poke above the mating surface on the crankcase, so I use a 1" X 1/2" X 9" ground bar to span the timing chest cavity so I can put the bar 1" high and measure down to the spindle shoulder, or the spindle shoulder with the appropriate shims on the spindle. In the past, the ET98/1 shims were .025" thick. I ordered some recently and they were .030" thick. This is not the end of the world, but if you want all your heights to be the same you need to be consistent. This is why I have a selection of shims from McMaster. Anyway, the ET173 shim is .071" thick. These shims go on the large idler and any small idler. So, it is best to measure down to all the spindles after you have placed the "under the steady plate" shims onto the spindle shoulders. If you do this, all the heights should be the same, because the steady plate should be flat.

When I do a Timing chest I use all Maughan spindles and the large idler mount. It is the large idler mount that sets the datum for all the other spindles because you cannot lower the large idler mount. Thus, the large idler mount with the ET173 on the spindle will set the max height for all the other spindles with their respective washers sitting on top. There is no need to worry about what goes on top of the steady plate, because you are focusing solely what is underneath the steady plate.

This should give you the basic idea about approaching timing chest assembly. Most good Vincent mechanics will do this one way or another. Most Vincents run fine with bent steady plates and do so for a long time. The long term worry is particles of aluminum in the oil and the possibility that a low spindle has the tension of the steady plate trying to pull it up or vise versa. These are real problems, but not the most urgent.