The photographs are very helpful. I have never seen a flange that connects to the gear input shaft like the one in your pictures. The screw that preloads the splines is not nearly as secure as a OEM design production flange. I would not use it.
With a production flange, when you tighten the pinch bolt the flange actually "wraps" around the gear input shaft. This prevents any motion between the flange splines and the input shaft splines. The pinch bolt also passes through a notch or groove in the input shaft and prevents the flange from pulling off just as long as the pinch bolt is in place.
I strongly urge that you purchase a OEM replacement flange assembly. First of all you will need to determine the diameter of the input shaft on your gear. The shaft is either 13/16 or 3/4 in diameter. The 13/16 input shaft was used until 1977. After that time it was 3/4 diameter. Hopefully your shaft is 3/4 because this flange assembly is a lot easier to find.
The following vehicles (with power steering) were produced with flexible couplings that will attach to a gear with a 3/4 input shaft:
1977 thru 1982 Chevrolet and GMC C/K (2 wheel and 4 wheel drive) Pickup Trucks
1977 and 1978 Camaro, Firebird, and Nova
1983 thru 1986 Chevrolet and GMC C (2 wheel drive only) Pickup Trucks
I have found a flexible coupling in the GM parts system that will connect to a ¾ input shaft. It is available through GM dealers. It is part number 7826542. It is fairly expensive (well over $100 list) but it is brand new. There are probably other A and F-car suppliers that have this flexible coupling available as well as Tom Lee of Lee Manufacturing in Sun Valley, California (818) 768-0371 at lower prices.
You will most likely require the attaching pinch bolt, nuts, and lock washers in order to attach your new flexible coupling. The correct special pinch bolt is GM #7807271. This bolt can be purchased from any GM dealer or from many automotive suppliers. Please note, if the flange on the steering column or the flange on the separate intermediate steering shaft (I-shaft) is also detachable, this same special pinch bolt is used to attach that flange as well.