There is no improvement to the lower belt mount on later cars. It's simply a nut welded to a piece of heavier gauge plate spot welded inside the inner sill.

You could fit a rod change floor to an earlier model shell if originality is not important to you. Main difference would be the trans tunnel shape being different and requiring a mounting bracket to be fabricated if you intend to run a remote style gearbox. You would also need to decide whether to retain the MK1 twin cable handbrake system or change to single cable as per later floor. 

I dont know the answer to your question about whether the attachments improved.  But I think you might find half floors available here in the USA, easier to ship.  See if MM or Seven has them.  After that, try M-Machine in the UK probably expensive to ship. Consider adding at least a partial roll cage with main hoop and down tubes to the rear wheel wells, attach your harness to the cage.

I've got a thoroughly rusted out Cooper S Mk.1 so I may as well start out with a whole new floor. Noting that new Mk.1 floors aren't stocked anywhere in the U.S. and shipping from the U.K. costs a fortune and they're not too available either, I'm thinking of substituting a Heritage floor made for a later model. Looking at the pictures where the seat belts attach looks more robust on the newer floor- Can anyone confirm this? And will it fit or am I just creating more problems?