[Dumpling]

The only way to be certain of anything after almost 50 years is to measure the bore and stroke.

Measuring the bore might present a problem without removing the heads, but you should be able to measure the stroke through the sparkplug holes if it really really concerns you.

If it's running and performing to your satisfaction, I 'd leave it alone and not worry about it until something breaks and needs fixing/replacing.

What were you told the displacement was when you bought it? If you convert to efi and need to set parameters that might be an important number to know.