I am working on installing seat belts and I have made some steel reinforcement plates for the mounting points. I was going to plug weld them onto the sheetmetal, but I got to thinking that maybe I should epoxy them instead. The strength of the seatbelt mount comes from the fact that it's behind the sheetmetal and would have to tear through it to fail. The plate interface with the sheetmetal would be in shear. The plates are 4.25X2.75" or about 11.7 square inches. Epoxies typically have a shear strength of 3500-5000 psi, so this should support 41K-58K pounds. Four 1/4" plug welds would only support about 10K pounds. So it seems like the epoxy is actually a lot stronger.
Is there any reason you would recommend NOT doing this? I have some high strength aircraft-grade epoxy that I'm thinking of using. The nice thing about it is that if I ever wanted to remove it, I think I could do it by heating it.