mike,
the bolts are 10x32 UNF, and there are 2 simple solutions that do not need machining. The first is to get six longer (say 25mm) 10x32 UNF bolts and secure the flanges using nut and washers. The second method is to use M5 x 0.8mm bolts directly into the existing 10x32 threads in the bottom flange.
M5 and 10x32 UNF have different thread forms, but are very close in pitch (31.7 tpi versus 32 tpi) and outside diameter (5.0mm versus 4.83mm). The slightly larger metric bolt will be held very securely, but try this on a piece of scrap first to satisfy yourself. This method (trick) was used by a famous Australian manufacturer of model aircraft diesel engines to prevent the compression tommy bar from loosening (10x32 UNF tommy bar into BA 2 threaded alloy cylinder head). Lastly, if you still wish to use a 10x32 UNF helicoil repair kit, it looks like you have enough room (see pic) for the required 13/64 (5.16mm) clearance drill. But practise on a piece of scrap first but unless you can very accurately drill the six holes on the required pitch circle diameter (PCD) in the lower flange, the result will not satisfactory, nor easy to repair.
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