It's a tough call on how much and what product to use in 50 yr old convertible isn't it? I mean, Top up, windows up and new rubber seals and it still isn't anywhere near as quiet as a 15-20 yr old car.
I'd be curious to hear the sound dampening that comes with any HVAC product because it's hard to fathom that it would be as good as a layer of the typical sound deadener. I basically agree with Dave though, the sticky/asphalt/rubber stuff is more directed at stopping sound wave travel along expanses of metal. It has a secondary factor of heat blockage.
The manufacturers clearly all state that for the sound dampening aspect, you only need around 40%-60% coverage, but you see people and car shows doing a full coverage all the time because there of that secondary aspect of heat blockage. (and sometimes it's other peoples money!)
I sucked it up and did the full coverage, way up into the toe-boards and Firewall, a couple of sheets in the door and back seat sides and trunk separation panel. THEN, because I'm a sucker for punishment, I did the extra noise/heat blockage you get from the foam-style sheets. (if I recall correctly, they are better at air-bound noise blockage and the heat blockage is secondary.)
All in all, I like Hushmat the best for both products, and they supply a ton of global auto manufacturers so that's gotta be worth something.
they aren't cheap though, so I bought some of their good stuff for the firewall/toeboard and tunnel, then bought Eastwood and DEI for everything else. (Eastwood was having a big sale)
**Note that Hushmat's foam product is adhesive backed. The stuff is fantastic! I believe the DEI foam product is NOT adhesive backed.
Here's my thread from just last year.