Correct way to do it is buy an oversize throttle shaft, made by SU for the purpose. Then you ream the body to be a neat fit on the shaft.
Don't just stick a drill thru it...
Bushing SU carb bodies to take a standard shaft is a factory reco operation.... if this is what you want, contact SU Midel in Sydney, maybe they can help. Easier to go find another better one...
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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.