Since I'm swapping a small block Ford into my BJ-7. this is a topic of particular interest and I''ve done a lot of research. There are different ways of doing this, some more expensive than others and some less noticeable. First, and cheapest, is to plug up all the holes in the firewall. Shine a drop light in the engine compartment and from the inside of the car, plug up with dum-dum, foam, etc, anywhere you can see light.
If you are doing a ground up restoration, a ceramic insulating paint like LizardSkin in the firewall and floor areas seems effective, although it wont give you a concours correct finish. If you want to save a few bucks, adding micro baloons to your primer is a good alternative. Check this thread:
lizard skin alternatives
Ceramic insulation of the exhaust header, downpipes and muffler will also pay off. Although most folks seem to go with JetHot coating, it seems SwainTech's (swaintech.com) White Lightning coating will give a greater reduction in underhood temp.
Next comes some type of insulation in the interior and you can spend a bunch of money on this depending how much you want to spend on noise reduction and heat reduction. Tsikuris
classics sells a nice pre cut Kool Mat kit
kool mat but it's kind of spendy. Kit Car Cobra guys seem to like the variety of products from
second skin
Or, if you want to go the cheap route, try products like FrostKing
frost king or Rflectix from the big box home improvement store.
Hope this helps,
Rick