Yoda'stoy, I have experience with Lizard Skin (and no affiliation) as well as mat-type sound deadeners, and I recommend it over any of the mat products. I'm an aircooled VW guy who is trying to get into the 40 game (anyone want to trade for a split bus?), and acvws are pretty freakin' loud. I did a bunch of research into sound deadening methods when I was restoring my '58 euro Beetle, and Lizard Skin was the clear winner on paper.
Now that I've USED it, I will be using it on EVERY vehicle I can going forward (it can pretty much only be used in a bare metal restoration scenario). I restored my Beetle to show quality, and didn't want the mat stuff ruining the look of the original pans, and I managed to get a hold of the engineer in charge of the Lizard Skin product line down in Houston. The product specifications suggest two coats of the thermal insulation followed by two coats of the acoustic insulation, then paint. You might be required to prime in between to reduce the texturing... I don't recall off the top of my head. This will leave a very heavy orange peel appearance. I told the engineer that I wanted it to look like nothing was there, and he said to apply three coats of each and sand between. I did this, and I am VERY pleased with the results. You still have the gross details of the pressings, and knocking on the pan is like knocking on a piece of wood (as opposed to the clang you'd otherwise expect). I helped restore a Super Beetle a year or so ago where the owner used some silver mat-type insulator. My first concern was water and stuff getting under the matting, the grit from the dirt that will undoubtedly accumulate rubbing through paint and the trapped water causing the pan to rust through VERY quickly (he totalled it before this could happen). It also wasn't a whole lot better than the pan was before the insulation... I was not impressed at all.
If you've got your 40 down to bare metal for painting purposes, Lizard Skin is an excellent choice. You have to apply a good epoxy primer first (Lizard Skin is water based), then Lizard skin the whole tub, then epoxy primer again (seal the top from any water getting in) then paint. You MIGHT be able to powder coat over it, but I'm not 100% sure of that one... Combined with a thorough coating of bedliner (which seems to be a common thing in this crowd, and would be my choice for durability) it would certainly be quiet enough inside to have a conversation without screaming, and maybe even in a fairly normal talking voice (I've never been in a 40, but it can't be much louder than an uninsulated bug/bus which will leave your ears ringing).
Again, no affiliation. I'm just coming out of lurking, so this is one of my first posts; I'm just speaking from personal experience with sound deadening products
