this filler neck leaking was a common problem. I have welded a few of them on different models including the alltrac for this same reason. They have since revised the design, the caps are also improved. I use the 18psi cap with no issues.
They now also have a high flow fan which is good for our hot running cars. I have not tried it out but the testing seems good. They have come a long way in terms of product quality over the past few years.
If anyone is interested in making a purchase or needs help getting there rad replaced please contact me, xiiimotorsports is an authorized vendor here and I deal with mishimoto regularly.