As someone who has done plenty of such work in the past, you really don't need as much vibration damper. What you need is 30-50% coverage on large, flat panels only and concentrate on certain key areas. Where panels are formed with creases or are small and can't resonate, it has no effect and only adds weight - so I would not for example apply it to the iner sills. It's isn't an effective sound barrier used as a continuous cover. In fact the P6 has fairly thick gauge panels, where this stuff really works is on thin door skins on something like a Honda. Closed cell foam, like the original jute stuff doesn't do much as it isn't dense enough to block sound, what it is useful for is as a decoupling layer under a barrier layer which is usually mass loaded vinyl. I've used stuff called TecSound 50 (which is a building material) in the past on top of 1/4" closed cell foam (aka cheap exercise mats).
Certainly, it's worth replacing the bitumen based damper Rover used because it dries out and no longer sticks to the panel with a modern, butyl product. After that, you really need to understand where noise actually comes from. On a P6 road noise overwhelmingly comes from the firewall where the suspension acts backwards against it. It's here you should start and also the transmission tunnel.
Good to see you've taped-up the gaping holes in the trans tunnel - the P6 cabin leaks a lot of air (and sound) through here and also in various places on the firewall and this is an easy win. Also goodness knows what noxious gases get in the car as well!
Unfortunately, after this the main source is wind noise as a function of aerodynamics (or lack thereof). If you want to reduce noise in any meaningful sense, then you need to look at improving the door/window seals - if these are old and flat, your best bet would be to replace them. I know it's a neat thing the close a P6 door with one finger with a neat click, but what you really want is a spongy door seal pushing back against you - it's doing its job. The worst offender of all is the deep step on the windscreen rubber, which would require a bit of reengineering to fix!