First off welcome to the site, make sure you stop by "general site info" and introduce yourself to everyone so they can welcome you in the "welcome" tab.
I am not positive a 400 cid came in 73, I know they did in 74 so I assume the answer is yes. There used to be a website called heritage in the tech pages, but I can not seem to find it, I e-mailed the owner, it has all the specs on your truck. You look on their site, look up your year of truck and it has all the options it came with. Hopefully it gets corrected, it is a nice tool to have access to.
As far as clutch's go, I only use stage 2 clutches now, stock one's don't seem to last long for me, especially in towing trucks.