unless you have a TIG, forget welding the valve cover. JB weld sucks compared to other epoxies out there. I dont know if you have one available, BUT I really like the NAPA expoxies.
As far as the exh mani, you can stick weld that sucker, and grind the weld down. This is only a temp fix, however. You should look into getting the 2G steel mani.
~Jason