Anyone recommend a very effective anti-heat soak spacer?

What kind of carb are you running? ThermoQuad? There's a much bigger selection of square-bore spacers out there than spread-bore and most of them are 1/2-1" thick. I put a 1/2" square bore wood fiber spacer made by Edelbrock on my 360 and it made a difference in hot weather. That also suffered from high underhood temps with uncoated headers, hopefully going to have them coated by a friend of mine for my 318 build I want to put back in.