In 1957, Mr. Duntov determined the "perfect" carburetor for the factory RACE version of the 283 was 380 CFM, and your truck requires 600 CFM ???

As far as thermal spacers are concerned, the two most common materials are fiber, and thermoplastic.

I am unfamiliar with the products you mention, but you might check max temperature ratings.

Jon

Good carburetion is fuelish hot air
The most expensive carburetor is the wrong one you attempt to modify.
If you truly believe "one size fits all," try walking a mile in your spouse's shoes!
The Carburetor Shop