Just wondering can i fit a late standard beetle floorpan to a 64 beetle without much hassle ?

yep its an easy swap.....you just need to modify (shorten) the body brackets on your 64 shell where it bolts to the pan under the rear guards.

On the later pans, the shock tower castings come up higher than the earlier pans.

This picture shows where the bracket is under the guard.

Its been done before a couple of mods as described, I have a IRS pan mated with a early front end for my "59, it was previously fitted to a '62 and fitted pretty well.