If I shave the heads of my Ecotec/L67 20 thou will I have a height clearance problem if so what will I need to get to over come this problem?

With an Ecotec/L67 engine, there should be sufficient tolerance in the lifter to be able to compensate for this. The only way to comfirm if this would be an issue though would be to measure the lifter crush to confirm if it's still within spec. Failing this you would need to get shorter pushrods to suit. To do this accurately though, you would need to determine the desired pushrod length using a pushrod length checking tool while targeting a desired lifter crush. Another option though would be to make up some spacer plates or use suitably sized precision round washers, underneath the rocker pedestals to compensate for the material taken off the heads.