The wood blocks I got in the kit were a hard wood. I don't know what type, but they weren't just a regular pine construction lumber. In my case, I think they'll last as long as I'll need them since I don't plan on parking it outside. They are under the cab, so they should be protected from any rain or other elements.
I suppose if you thought you would want something better, you could probably use some UHMW (ultra high moleculer weight) plastic. Usually you can buy it in various sizes and then you could cut it to the size you want with regular tools. This would probably outlast the truck and would have similar properties as the wood blocks.
This is kind of an expensive option, but just to show what I'm talking about:
High Density Polyethylene
