Lots of opinions around of what to do and what not to do.

K.I.S.S

On the intake side, enlarge the thing as much as you like, but dont worry about polishing the port. Energy derived from fuel is very highly dependant on the atomisation of it. The finer you can atomise the fuel, the better and more efficient it will burn. Leaving the port surface slightly rough, aids in turbulence which helps atomise fuel better. Especially since the manifold is so short on Mini's. Just common sense.

It's just as simple for the exhaust side. If you need to get good air in the engine, the quicker you can get the burnt air out, the better.
This is where you want to maximise the port size, and polish it to make flow as smooth and fast as possible.

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"He's always been a dickhead, He always is a dickhead, He will always be a dickhead. So stop complaining when he acts like one."

Just be careful- it's easy to hit the pushrod holes (yeah, they can be sleeved) or break through in the Y part below the inlet valves. Vizard was right..

I had to gas weld mine, it's so thin, but that's sweet- I'm a welder..

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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.