Thanks to Myron, Michael, Greg and Les...
GRANT PART #6504 for Fiat/Bertone X1/9 and 128s
GRANT PART #6506 for Fiat 124/2000s
These kits will accept Grant and APC five bolt steering wheels. There are also adapters available through other venders for a 5 to 6 bolt conversion... I have not the source for these though. These kits will NOT acept the old Superior Industries or Grant 3 bolt wheels.
Michael Wilson writes:
GRANT Part #4009 for the X1/9 is for putting a Momo wheel on a Grant hub, converting the Grant 5 bolt pattern to the Momo 6 bolt pattern. It is a Grant part, not a Momo part. The Momo hub kit has been long discontinued, so I was unable to find out the part number for it. What started me reading the Best Of article was that there was a Momo hub on ebay, part number 4013, and it was listed for a Fiat and Ferrari. (I am unsure if the Momo 4013 kit will work on an X1/9) So, these days, if you want to install an aftermarket wheel, you pretty much have to buy the Grant hub, and if you want to install a Momo wheel, you must use the adaptor to get from the 5 bolt pattern to 6 bolts. I think that Grant also sells bolt pattern adaptors for patterns other than the Momo wheels. I bought mine from Summit Racing, where they offer free shipping in the USA for all orders.
For 6 hole MOMO wheels and others. Myron states he used a MOMO kit for a 124 on an X1/9... Which makes one wonder what the differences are in the GRANT kits... I imagine the steering shaft sizes and splines would have to be identical... and just the external "cup" size would be different... but not necessarily a prohibiting factor in its use or safety. If Myron is correct... then maybe the hub and splines on a Fiat and a Ferrari are identical... and interchangable between the 124s and X's.
Les suggests looking into Bruce's Part's Bin for still another alternative source for aftermarket wheels and installation kits. Bruce appears to say that these wheels (and kits?) fit both the 124 and X1/9... and appear to be a MOMO 6 hole configuration for the most part.
Hopefully we'll get some objective info soon and I'll update this post.