I've had a devil of a time finding any info on these rare Canfield LT1 heads and one of particular frustration has been "what spark plug does it use?". I even called Canfield and the guy that made them (John) didn't know jack about the heads. Not chamber CC, not spark plug, not guideplates, not head gaskets. Nada. I said, "do you not even have some old literature about them?" and he immediately said, "nope".

Gee... now I understand why they didn't sell very well.

Anyway, I EVENTUALLY found out they use a .750 reach(alternately referred to as 3/4" or 19mm), the thread size is 14mm. Next was the heat range determination. The way that the different manufacturers refer to the heat index can differ. Autolite and Champion use low numbers for a cold plug and high numbers for high heat. NGK and others use the opposite.

Example:
Autolite 103 is a heat range '3', aka 'cold'.
Autolite 106 is a heat range '6', aka warmer (actually a stock heat range for an LT1)
The last number of an autolite plug is the heat range, which can go from a '0' to a '7'

Here is a great reference to look up the various Autolite plugs. If you find your ideal plug here but want a different brand , you can use a plug cross reference.
Hope this helps you !
Just drop down/select your key criteria (reach, thread, gasket/taper, etc) and hit the 'find!' button. They'll show you an organized list of their platinum, their premium and their regular plugs with prices that meet your criteria.

Autolite plug search

 

Torch (china) makes some of the aftermarket Autolites, so watch out for those.