Shamrage, it could be modified to accept 3/8" rods, it would require a complete rebuild of the kit, and honestly 3/8" is a bit overkill.
As for the current kit the nuts would not fit inside the caps, the nut for the bottom would be to large to fit in the tube that helps keep the nut from spinning.
It took 112hrs of modeling, printing, testing, remodeling, printing and testing for this kit. I currently have 4 other projects going on so it would be a while before I could come back and revisit modifying this kit to 3/8" rods. Sorry.