Yeah that #12 should be the one, but you have to make sure you still have the 2 bottom bolts that hold that pad on the floor. Like you said, the person that upgraded it to a modern style pedal may have eliminated the bolts and holes.
I went with Lokar pedals and I will say the difference was amazing, not to mention it looks great. You probably have the setup already in place to plug and play. Just throwing it out there, it's definitely not cheap.
Here's the throttle pedal I used:
https://www.speedwaymotors.com/Lokar...bly,67681.html
Of course you have to add the throttle cable... and then you'll want to use their kickdown cable, carburetor bracket... and then you're broke, wife screaming at you... I'll stop now.