EARLY A booster

Hey all, tried the search thing and there's just too much to go through. I currently have a 73 manual disc setup on my 65 Barracuda. The car stops great but it has a rock hard petal and I'd like to soften it up a bit. I have 2 questions:
1st is there a difference in master cylinder force (piston Dia.) and can I tell what I have.
2nd what booster would work and what (if any) modifications would be needed to add one?