I'm not entirely sure I understand the problem but I just converted my SE (which I thought was the same spline pattern as your later car, but maybe not) to a Momo wheel.
I used the grant 4509 adapter and this Grant to Momo/Nardi/Etc adapter made by ForeverSharp:
3 Hole Grant to 6 Hole Momo Nardi Personal NRG Sparco Conversion Adapter Black | eBay
...They come in a variety of depths, not sure what you need for your Nardi.
I cut the lower part of the two "nubs" on the grant adapter off (this is documented elsewhere on this forum). I turned it in a lathe but you could just grind them.
I left the clock spring intact, added the 2-Ohm resistor to the airbag connector and cut the blade connector on the Momo button so that the narrower OEM horn lead would fit.
I even left the OEM white plastic turn signal canceler in place. I had to play with the indexing of the Grant part so that the two "legs" on the canceller would interface with the indentations in the backside. It works OK but sometimes it missed cancelling right turns. At some point I'll 3D print a better replacement.
The (stupid) Momo horn button doesn't work with the above mounting adapter, I had to 3D print a spacer ring to make it fit. I also 3D printed a shroud that fits nicely over the clockspring.
I installed the Momo Jet 320mm wheel. It's also available in a 350mm version. The 320 is an offset wheel, having the same diameter as the 350 at the top, so low speed turning isn't overly difficult.
Overall I like the wheel, I love the extra space for my knees. The small diameter makes the steering feel a little "twitchy" at speed and it did look a little odd until I got used to it. I occasionally autocross the car so it's perfect for that. I think most people who don't need the extra legroom would be happier with the larger version.