The use of plastic, which is a non renewable product made from non renewable resources is not necessary and will do little to help achieve what you want, so in addition that would be a waste of your money. If any heavy metals do leach from your pressure treated wood they will not migrate very far into the garden soil, all of the studies I have seen indicate at most, maybe, possibly, 6 inches. Then even if those heavy metals are in the soil the plants growing there will not, in a good healthy soil, uptake them and if any root crops are grown there and are washed well before eating any heavy metals that might accumulate on that food surface will be washed off.
If you have the URLs of the sites that do show that plastic sheeting actually does leach toxics into the soil (I know there are some types that do) I'd like to know about them since I've not found a reliable one yet.

You already got the wood for free. Now you want it to last forever. Some time ago there was a discussion here about the proper use of plastics in the garden. The upshot was that there is no proper way to use plastics in the garden.

If you want to garden with an organic program, there are some bullets you'll have to bite. Untreated wood rots. We have used treated landscape timbers as a border and have had no problems with plants nearby. We don't grow anything edible in those beds. However, if you have to cut the wood, that sawdust will sterilize your soil for years to come (unfortunately this is the voice of experience).

If you can get all the treated lumber out of your project and use only the concrete tainted wood, you should be fine. You could simply paint the wood. Of course paint is liquefied plastic so you're back to plastic again.

Is it too late to change the border to cinder blocks?

How old is the pressure treated lumber? I know it isn't "organic" to use treated wood but if your goal is helping the environment and not specifically "being organic" by it's rigid definition, you can probably use the treated wood with no ill effects.

I hear a lot of people talk about "chemicals leaching into the soil from treated lumber" but I find that very few of those people can actually specify what chemicals are so dangerous. Even the small group of folks who can identify the harmful chamical [CCA] are often unaware that it hasn't been used to treat wood for over half a decade. That's right, they haven't used Chromated Copper Arsenate (the stuff that contained arsenic and gave treated lumber a bad name) to treat wood since 2003. Any wood treated after 2003 used either Amine Copper Quat or Copper Azone to treat wood. Neither of these contains arsenic or has been show to leach any undesirable chemicals into the surrounding soil.

It may be poor form to advocate use of non-organic materials in the organics forum but I feel that presenting both sides of an argument is the only way to allow someone to make a truely informed decision about how to set up their garden.

Good luck and happy growing whatever you decide to do.

my problem with any information on any "leaching" of chemicals from wood or plastic is that we don't know what's bad until someone discovers what's bad. (you think BPA is the last problem with -and here's a good one- "food grade" plastic ? also, who's doing the testing and who's funding the research? thank you, but I'll trust only what the earth has made! having said that, i do use weed mats and plastic containers because it's that or spend a fortune or just not grow as much. i use them, but I don't like it. life is full of compromises.