I appreciate the great experiences shared. Very helpful.
Right now here is where I'm leaning -
Floors -
1 - clean up much better - adhesive removal from old original yellow insulation
2 - since I have full access, I think it would be a good opportunity to spray the floor with rust inhibitor
3 - add sound deadener floor kit from ACC -
https://www.accmats.com/commerce/1972/Ford/Mustang/Sound Deadener/10236/ - This kit is precut and includes the trunk which is nice.
4 - add mass cut pile carpet using suggestions from 72mokk1 on installation.
https://www.cjponyparts.com/acc-car...ack-black-fastback-mach-1-1971-1973/p/MC53-V/ I need to dig into Rock Auto to find the equivalent (I really hate their website). I'll probably go with cut-pile since this is middle of the road ACC quality (one higher and one lower in quality)
Doors -
1 - add some butyl rubber mass damping on the inside door skins (I have some already). It will be a pain but I can put strips in through the access holes if I take my time.
I realize this doesn't align with some people's dyanmat 100% coverage philosophy but I think with doing a both the sound deadener from accmats and the mass carpet on the floor it should be quite affective.
Thanks for the insights -