LD340 MODIFICATION OR NOT?

I'd cut it for the spreadbore and at the same time smooth the transitions under the mounting plate into the runner. Port match as well, of course. Yes they don't make them anymore, but they are a car part, not art artifact. If someone down the road wants to restore it, takes an hour or two of welding and cutting to get back to square bore.
The other option would be to use an adapter which is fine too, if you have the hood clearance, and has been done for decades