i am not qualified to say for definate why you should or shouldnt modify in this way. But i do find it strange when its assumed that a company that spends millions in research has got a design so badly wrong that they need to alter it so simply. Surely logic would make you ask why it has been done in such a way. Why did they go to the bother "and expense" more to the point? if it could have been done cheaper and easier, i would think they would have done so surely?
But if you must, i would guess either cutting a new plate from scratch with no oil holes, or block with suitable bolts , washers and sealent.
I would guess also that it would be a good idea to sight the new cooler behind the radiator and fans somewhere with max air flow and run the oil lines (goodridge braided or similar as said) to where the old oil pipes exit by the filter, and junk the old oil lines altogether.
I would also guess that its very difficult to find the correct size cooler to give the best cooling rate, too small or too big ? Where to start?
Giving your self a mare i would think.