RE: Ford Lehman coolant reservoir cap repair
If this is anything like the kit that replaces the stock neck insert and cap with an automotive-style recovery neck and cap, the directions call for the stock nec insert to be cut out VERY carefully with a small hacksaw by cutting though the stock brass neck insert longitudinally. Remove the stock insert and install the replacement insert using JB Weld (that's what the instructions call for).
If what you're doing is not like this particular process then I would call Bob or Brian Smith at American Diesel. The know more about the Lehman engines than God (they are the ones who supply the conversion recovery system, for example).