34 steering box swap for 37,38 or 39?

[trevo]

Hi Ford Men, I have Coker tyres and kelsey Hayes style wider wheels which look good gives good handling but provides for heavy steering over twisty winding roads not good for my shoulders and neck. It has been suggested that I fit a 37, 38 or 39 steering box with the better 17.1 ratio and maintains the light switching is this an easy one if so does anyone have a LH box I could purchase your comments please.