which phenolic spacer should i get?

[1972BlueC20]

Hi Guys!

I have a 1972 C20 (350 engine) with the stock intake manifold, running a edelbrock 1406 (I believe) carb. I'm having issues with hard starts once the truck is hot. Its never an issue of the truck not starting, but its just harder than when it's cold. I'd like to fix this issue so I don't wear out my starter.

There are many spacers available so it's hard for me to choose.

I've already got the edelbrock adapter plate to convert from spread bore to square bore. I'm assuming the spacer would go on top of the adapter, is this correct?

Can anyone recommend a good spacer? Thickness? Material?