Converting '68 120 mph Rallye Speedo to 150 mph

They are not that uncommon why not just find a 150 speedo for 68 up? The only difference between the 68 and 69-71 is the direction of the numbers, change the face decal and none would know and you can also retain your tripometer. What were you quoted to do the 120 conversion?
The 67 is a one year only has the same face as a 68 but it has a screw on cable where all the others uses the push on type.