1970 Valiant Disc Brake Conversion

Buy what appeals to you, but consider using a kit based on stock parts. Calipers, pads and rotors that were used on production cars will be easier to find than proprietary parts used by companies like SSBC or Wilwood. When they discontinue a line of products or go out of business, you're up the creek for wear parts. I know a guy who used a SSBC kit because they were local to here. They went belly up, and he was stuck for rotors. He changed the entire setup using stock spindles and caliper brackets. To add insult to injury, SSBC was reborn, and might have rotors again.
My first stop would be Dr Diff, and I'd check Scarebird, too. Both base kits on parts you can get anywhere.