"Eureka." Or "Mr. Watson, Come here." That`s how I feel today.
Thanks for all your thoughts and ideas guys. After more than a couple of hours fiddling, pondering, writing, reading, and more fiddling, I have the spring and its snap-ring in place.
While I think there is some merit in SteveJ's suggestion of compressing the spring with fishing leader, the only fishing line I have is about 40 years old, and it was not up to holding a steel spring in compression.
I tried Cap't Obvious's suggested of carrying the clip into position by squeezing it into a tube or pipe, then compressing the spring using the resultant assembly, but as you can see from the first photo attached, the clip was simply too big to fit into a 3/8" i.d. piece of pipe.
Finally, I took my snap-ring pliers, and set the pins in for maximum exposed length (see photo), as the groove is 16 mm. below the outer end of the cylinder. On this latter point I should mention that the factory service manual drawing BR-8 is just about exactly to scale. It took many tries, and in fact the notches on first spring clip I used simply wore out. Fortunately I had another old proportioning valve, courtesy of ZKars (thanks Jim), and was able to salvage its spring clip. With the spring in place, I set the clip on top and pressed down with the snap-ring pliers, while simultaneously forcing the opposite side down with a small pin punch. After xx tries, it finally set.
So it`s done. But having said that, I remain a bit concerned that the inevitable scratches on the side of the piston may cause some grief down the road, although it appears to me that the real work of the piston all happens at the bottom end.
With respect to the repair kit from Motorsport, i think I`ll drop those folks a line and suggest that for $1