Converting the post-80" S1 pedal system to the series 2/2a/3 pendant pedal system is a very common conversion.
The parts for both the brake and clutch are readily available, the S2/3 firewall is basically the same angles as the S1 so all you need to do is add the reinforcings of the S2 top footwell and blank the holes in the floor.
You need to swap the hydraulic clutch mechanism from a s2/2a onto your s1 and if yo are using an original 2 litre engine you merely use the 6cyl S2a mechanism.
Some people even go to the extent of using the 6cyl/V8 pedal box with booster. This will probably require some trimming of the top of your front RHS mudguard.
Remember your master cylinder should be of the same type as your brakes. If you retain the 10" brakes you use the short wheelbase master cylinder and if you have 11" brakes then you use the 109" master cylinder or mastervac.
Diana

