If I understand your question correctly, the aluminum casting is different. The 1969 casting has a hole on the left side for the clutch shaft to pass through. Of course the clutch shaft is different as it goes out the left side on the '69, while the '65-'68 shafts go out the right side.
Does this help? Are you converting to hydralic clutch in a early chasis? If so I have a complete '69 set-up for sale, with everything needed for the conversion.
As both Brad and George point out, early boxes have the clutch linkage coming out the right side for mechanical actuation, while later ones have the clutch shaft & lever out the left side to actuate the clutch master cylinder.
Once you put a hole in the frame rail, a hydraulic clutch pedal box will bolt right into a mechanical clutch chassis. The mounting holes for the two styles of boxes are the same (if that was your question).
One bit of trivia: The pedals themselves differ between models. Dunno if this is a mechanical vs hydraulic clutch thing, or a spider vs GT vs sedan thing. But, pedals from different cars (and I am referring to just standing pedal cars - of course hanging pedals are different) will differ in length and the shape of the bend. Anyone know why (I certainly don't)? I learned this when I installed a '69 pedalbox & pedals into my '65 Sprint GT. The cut-outs in the '65 sheetmetal plate that covers the pedalbox would not accommodate the '69 pedals.
Mind if I ask what parts one needs to do the conversion properly?
I think the clutch on my car was converted, however the pedal is very very stiff, I was thinking that it was because the clutch was going but a mechanic told me it was probably due to the pedal not being correct.
Also more reciently, the clutch has been slipping, almost like the pedal looses pressure, and if I pump it, it gains its pressure.
GeoffSprintGT had asked: "what parts one needs to do the conversion properly?"
Well, basicly you need the pedal box from a '69 (of course with all the pedals and rods), the clutch MC, and the rigid lines that plumb the MC to the reservoir and to the clutch slave. You will also need either a 2nd reservior, or one with two bungs (one for the brake, the second for the clutch). And, you need to modify your frame rail by cutting the large hole that the clutch rod to passes through, as well as drill & reinforce the holes that the MC bolts through.
He went on to say: "I think the clutch on my car was converted, however the pedal is very very stiff, I was thinking that it was because the clutch was going but a mechanic told me it was probably due to the pedal not being correct. Also more reciently, the clutch has been slipping, almost like the pedal looses pressure, and if I pump it, it gains its pressure."
Sorry, don't have enough information to give a meaningful reply. The difference in pedal length that I observed - about 1/4" - wouldn't be enough to result in stiff operation if a shorter pedal were substituted. I suppose the first thing to diagnose is whether the stiffness is due to a mechanical problem (in the pedal box vs. in the bellhousing) or a hydraulic one (e.g., flex hose swollen almost shut).