I'll scrape off the paint and look.

The large pulley in the photo is marked GM 3790356. From the face of the pulley to the back edge is a little over two and three-quarter inches. It's not 4 inches deep like what you need. Sorry for the bad news.

Lugnutz -

Thank you for looking. My truck is at the paint shop and will be for several weeks as it waits its turn. I'll keep looking for a pulley. There are a couple on Ebay that I have been watching (for a price reduction).... My Dad used to say "I'm not cheap - but I'm reasonable"!! I appreciate all of your responses and will keep up updated on my progress. Tim-o

Just a brief update. It took me almost three years to get off my wallet and buy an aftermarket air system from Old Air for the '66 C10 250 6 cylinder. I had the system charged last week and the air blows a cool 43 degrees. I worked a couple of months on the install following the detailed instructions from Old Air and adding an electric fan to the install. I must admit, the project stretched my mechanical abilities (you have all read my posts) . I did have help - in that John Price from Old Air answered my MANY questions - no matter how trivial - by email and on the phone. All in all - it was a fun project and I got to know my truck much better. I guess what I'm saying - if I can accomplish the install - almost anyone can. Tim-o

Do you have a normal size distributor cap, or an HEI? I’m curious if the brackets work with an HEI dizzy.

My truck has the stock distributor. I suggest you contact John Price and ask him. He’s a real nice guy... very helpful...