I am sure that this discussion as come up in the past. My 68 442 convertible as a 455 and I am getting a lot of heat coming in from the engine compartment. None of the original insulation as been replaced. Would I be better off going with the standard factory replacement from Fusick's or is there a better more insulating material out there to substitute both the "under dash insulation" and "sound deadener package"? What are the best insulation materials of the same thickness available on the market?

 

You would be better off with dynamat, fatmat, or similar products for insulating. The factory used thin strips in a few places; you need to do the whole firewall and floorboards to insulate properly.
I did my car completely in dynamat and I do not seem to get much heat fromt he firewall. She rides a lot quieter, too. My interior resto thread gives details on the job.

 

I also put header tape on my exhaust when I did my insulation to keep heat off the floor. The insulation made a big difference in the noise level with windows up.

 

Great idea but I have the stock 68 exhaust manifolds X & Y, were would I put it?

 

Watch that exhaust tape, it causes the pipes to super heat on the outside surface and shortens there life by a lot. I bought a used stainless exhaust system for a Harley that was rapped in that stuff and the pipes where so cooked you couldn't get them to polish back to there stainless color anymore.
I used Hush mat in my car but I didn't do the entire firewall just up to the bottom of the air box. It helped a lot with noise not as much with heat.

 

I'd heard that Jag. I also put it on to keep the heat off the starter. W-27 if your pipes come close to the floor you could wrap them there. New cars have heat shields on them to block some of the exhaust pipe heat from the cabin.

 

I used dynamat as well, helped a lot with heat and noise.

 

Don't forget to look for sources of heat transferred by convection (air flow). First, make sure your heater controls are working correctly and that's not contributing. Next, make sure engine heat isn't coming up through a hole in the trans tunnel or elsewhere. Under the hood is a high-pressure area; the hot air will find any leak into the cabin.