Hi Kev
Found the info you need 4.3 ratio transmission low gear (tractor side) teeth should be 21 or 27 on 7.5 ratio trans, Lift drive housing teeth should be ,idler gear 26 or 23 teeth on 7.5 ratio, , pump Drive shaft gear should be the same for either speed at 29 teeth.
The Idler gear supports are different lengths also, the 4.3 ratio is 5 3/4 inch and the 7.5 is shorter at 5 1/4 inch approximate overall length( from radius'd end to far end of threads ).
The gear housing is the same for both ratio's ,Its just the 2 gears and idler support you need to make it work. The shaft length for smiths is the same for both ratio's at 13.84 inch. if you happened to go to a varley top ( 2 lever style) the shaft is longer at 18.44 inch, but use the same drive gears/ idler supports, and castings as above for the ratio's .
Hope that helps you a bit

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Regards Robert
A Fordson is for life not just for Christmas !.