Finished welding and fettling the last window frame today.  However I need to check the other for dimensional accuracy, as at last one is out by 1/4″ at the top.  This is easily rectified by cutting a slice out of the top horizontal and strapping the sides with a ratchet strap with a jig bar of the correct size between the uprights.    With this setup I was able to weld the top horizontal in the correct position.  I think this problem is only likely to affect the first two or thee that I made, so nearly there.