Two things concern me regarding redrilling the Milano hubs: If the holes aren't absolutely precisely drilled, the wheels will be out of round. Secondly, since the 5 x 98/4 x 98 openings holes will be so close one another, it seems to me that the four holes should be welded up, then machined down.
I reckon it's time to harp on the wheel stuff again. Biba, you being a fella who drives Alfetta GT as a daily driver, I just have to tip my hat. Since you're going with the Milano set-up in front I can only assume where talking about the rear here. Whether you drill or not, the wheel studs should not be what center the wheel (though we talked about this already w/Alfettaparts)
The HUB is what centers the wheel AND takes the loads. Case in point. This 16"x7", 5x100 rim has a 73mm bore.
Aftermarket wheels usually have bigger-than-necessary bore so that they'll fit many different cars. however, they require the use of Hub Centric Centering Rings to be safe.
If you look back at the first pic (white arrow), you'll see that the machinist started to drill these 5x100 rims to 5x98. He stopped. The wall between the bore and the stud holes became too thin and most likely would crack. Put enough load on it and you'll be likely to ruin yours or someone elses day.
The solution in this case? Use the proper hub-centric rings, and wobble bolts. Works like a charm. Her'es a pic with the centering ring dropped-in so you get the picture. If it weren't there, then the studs would bear the load and you would most likely be off-center so that high speed would be dangerous (and like driving a chuck-wagon across the Chisum trail!).
Rather than having to pull your hubs, remove existing studs (so you can use your 20mm spacer), press-in longer ones, then drill-out the wheels to fit 4x98, Why didn't you just order 20 or 25mm Wheel Adapters?
In 20mm the existing studs and nuts would protrude from the adapter, so you would have to make sure the wheel has cavities in the back to accomodate. Most do.
Note that the wheel adapter also has a protruding lip (hub) to fit the wheel bore that you plan to fit. The lugs job is to clamp it on.