Due to popular demand (well, I think one person asked) I'll outline what I did and will do to control the noise in my '68 122.
When I first got the car (7 yrs ago) I had to shout to speak with a pasanger, and this with the exhaust not being all that compromised at the time. Now it seems almost like a "normal" car when riding, except when going 70 or faster when the exhaust starts to protest.
Two of the pricipal substances I've used to deaden sound are dynomat and that spongy self adhering stuff. Both were purchased from JC Whitney.
Before I had the carpet installed I stripped out whatever was on the floor down to the metal. I had already repaired the floors so I painted them with some kind of rustolium paint. Then I covered the entire floor with a layer of the dynomat, followed by a layer of the spongy stuff. Then the upholsterer put down a layer of "sound deadening" mat and then the carpet.
That's the floor.
By the way in the interest of puddle control on the front floors I applyed window and door silicon sealant around the windshield (that is, on each side of the winshield strip chrome and around the whole windshield. It works pretty good, although tends to need freshening up once and a while.) and bought some pretty study rubber mats, for when it does drip.
The inside of the trunk and hood lids both recieved a layer of the sound deadening sponge, an inch thick. I took them off and put them inverted on a table and stuck the sponge which is self-adhereing, but not to be trusted)down with industrial strength glue from the hardware store. The front of the trunk also got the "sponge treatment, as did the inside of both doors.
Before I put in a new headliner a few years ago I glued (and ironed!) some dynomat to the ceiling.
The front windows are an ongong mission. So far I have installed new rubber around the vent window and got some of those security latches from IPD, which keep the vents closed and secure. I glued in new felt in the runners of the front windows. They're kind of hand crafted from a Whitney products and do, in fact, stop that irritating wind sound when riding up the freeway if I be sure to put some extra pressure on the cranks. I want to update the runners with a better material.
When I install my M41 w/OD next summer I'm going to "insulate" that whole area. I just haven't figured out exactly how yet.
Speaking of which, can anyone tell me how to cut the hole in a 122 when I convert from the long shifter to the short. The M41 is from a P1800 and of course has an extended cover. I know I can switch plates, but I think the short shifter will be kinda cool.