Hey guys:
These are the repair techniques I attempted on the original tank:
1. Drilled a small hole at the end of each crack to halt further propagation. Opened the crack with a fine Dremel bit so sealant could mushroom into opening (instead of just sitting on surface). Thoroughly sanded surface and cleaned with acetone.
2. Applied JB Weld.
Result after assembly: JB Weld cracked - tank leaked.
3. Purchased two-part Permatex Fuel Tank Sealant (comes in a stick). Cleaned and prepared tank as before. Applied per instructions. Applied some air to tank to simulate flex from vapor pressure.
Result: Sealant bond with surface cracked before installation.
4. Purchased Permatex epoxy resin / fiberglass tank patch kit. Installed nicely, passed pressure test, allows some flexing. I thought this was it!
Result after assembly: Fumes in car. Applying air pressure to filler neck results in hiss from top of tank. I am assuming this patch failed too.
During this time I was trying to source a good tank, and found all to have the same cracks.
This is why I am investigating a solder repair or switch to the plastic tank.
Jimmyvirus: I am researching the swap right now, but it looks like I can use the 85.5 an newer 944 tank in place of the steel unit. I will need to acquire all the associated vent lines, filler neck, and mounting straps to make it happen. If anyone knows otherwise - let me know
Cheers, Neil