Okies lets see if my brain is up to math tonite hehe
3.375 yds long x 42 wide = 135'' x 42''
if you cut the 135 in 2 at 67.5 in and sew them together on the long sides *the 67.5'' sides*
it should come out to approx 67.5'' x 83''
you will only have 1 seam and it will be plenty big enough for your quilt
hope that makes sense if not lets try it this way hehe
fold it in half from end to end long ways and cut at the fold
this should give you 2 pieces measuring approx 67.5''x 42''
sew these two pieces together along the long sides (will be selvage sides)
and it should give you a rectangle measuring approx 67.5''x83''