Mark, whilst ‘line drilling’ of bearings can be carried out as you describe it is one of diminishing precision unless the set up and machine are rigid and accurate. The bearings in such an engine should not just be drilled either but bored to a good fit and surface finish to take the shaft with the right tolerance. If the bed can be set up on the lathe and the bearings pre drilled then a between centres boring bar could do the job with much more ease and accuracy throughout the four bearings.

Personally, given that the housings for the bearings in the bedplate are in line and uniform and the bearings a good fit in the housings, I would do each bearing individually on a fixture to a set dimension and fit separately tweaking until a good running fit is acquired. This is the method I have done on the marine engine described elsewhere – that just has three bearings though.

There’s a lot at stake so although more work initially, to get this part as accurate and as smooth running as possible will pay off further down the line as the build progresses.

Further to my previous post Mark the drilling of gunmetal, indeed any of the lower end bronzes really, requires razor sharp tools for the best result so a freshly ground drill is the first thing in order for success.

If a tool is nothing less than sharp even if only used once before and particularly so if previously used on on steel. It will not cut very well but tend to ‘push’ the material – achieving a decent surface can be quite a fraught exercise if the cutters are not really sharp.
It is far better to bore bronze for bearings either in the lathe or using a boring head on the mill, again with freshly ground HSS tooling for best results

Gunmetal itself, as no doubt you’ve previously found is easilly distorted so it requires holding with the minimum pressure to achieve the machining involved, inserting a slip of copy paper between work piece and table/ vice , etc will improve grip considerably for a given clamping force

Hope the above is helpful to you

I have had no trouble drilling SAE660 leaded gunmetal using decent sharp drills, I found it easier than brass and copper. I would drill undersize and finish bore unless I had a solid carbide drill that was the right size. If possible use a mill or a lathe for the drilling which give more control than a pillar drill.