The correct shift knob in the pre 1983 spiders were those:

43mm black hard polished bakelite with metal thread
If anybody is interested I have some of those in NOS left in my stock...
They are similar to those installed in Fiat Dino 2000 and Ferrari 365 400 412

Ferrari 400i interior
Same item was used for a while in Fiat X1/9 and 125
http://xwebforums.org/showthread.php?t=21495
If you are interested you can send me an email:
mrtubeamp at web.de
I also sell a few of them on ebay,
http://www.ebay.de/itm/161036224126?ssP ... 1555.l2649
its up to you to restore
the nice original "Ferrari" look of your Fiat shiftstick, you wont find those
NOS somewhere else.
best regards