Those must be some suckass bolts if you're stripping the heads out. If they are SST then that's just the way it is, SST is hard and soft all at the same time, it's a weird metal. Most SST bolts are made from 17-8 SST, which has marginal corrosion resistance. Use standard black oxide or grade 5 or 8. What size are they? Length, pitch and diameter. I have several fastener companies I use and can probably get you something of better quality.
as far as testing it will be difficult to put the core in something and keeping it from imploding when you pull a vacuum on it. You wouldn't happen to have (access to) a referigerent pump either air powered or motor powered do you? If you do then you could place a couple of sheets of steel or 1/4" plexiglass/lexan/acrylic in top and bottom of the core holder and draw a vacuum and see where the leak is. You can find the plastic at a plastics place, they usually sell scrap by the pound. I use a place called Regal Plastics in Houston. In fact I have a ton of it at the shop. I'd be willing to send you some, just pay the shipping and let me know the dimensions you need.
if you have access to an air compressor you can buy an air powered vacuum pump for like $12 from harbor freight and it serves double duty when you need to work on AC.
just smear some grease around the upper and lower part and slap a piece on either side and draw a vacuum on it like I described earlier and you will see where the leak is.
Jeff
