The exotic metal plugs are fine for the highly controlled fuel delivery of an EFI car.
On a carburetor equipped car such as ours, they do not do well, and will foul as yours have.
Autolite 25 is a good start for a basic resistor plug. Cheap and historically correct for a carbed Cleveland.
Now, you could actualy have an oil problem too.
But as for plugs, this is where you want to go.
Larry
PANTERA INTERNATIONAL
A DE TOMASO CAR CLUB
Presents the De Tomaso Forums
The On-Line Meeting Place for De Tomaso Owners and Enthusiasts From Around the World
Clicking on the banner will take you to the sponsor's website.