documentclass{letter}
begin{document}
thispagestyle{empty}

signature{Nathan S. Lachenmyer}
longindentation=0pt
letraggedleftraggedright
vfill

begin{letter}{
Senior Staff Recruiter \
XYZ Corporation \
Rt. 56 \
Anytown, New Jersey 05867}

%This is my header
centerline{huge bf sc Nathan S. Lachenmyer}
begin{center}
line(1,0){450}
end{center}

%letter begins here
opening{Dear Person:}

Letter goes here

closing{Sincerely Yours,}
end{letter}
end{document}

documentclass{letter}
usepackage{showframe}% http://ctan.org/pkg/showframe
begin{document}
thispagestyle{empty}

signature{Nathan S. Lachenmyer}
longindentation=0pt
letraggedleftraggedright

begin{letter}{%
Senior Staff Recruiter \
XYZ Corporation \
Rt. 56 \
Anytown, New Jersey 05867}

vspace*{-10baselineskip}% Correct for vertical displacement

%This is my header
centerline{huge bf sc Nathan S. Lachenmyer} par
%begin{center}
%line(1,0){450}
%end{center}
hrulefill

%letter begins here
opening{Dear Person:}

Letter goes here

closing{Sincerely Yours,}
end{letter}

end{document}

makeatletter
  def@texttop{}
makeatother